First, it would be a mistake to skip the preface to the Philosophical Investigations, but I will note just a few things about it: 1. like the Tractatus, it contains thoughts but is not meant to tell people what to think (rather, Wittgenstein says he wants to "stimulate someone to thoughts of his own"), 2. speaking of ownership, Wittgenstein says that: "If my remarks do not bear a stamp which marks them as mine, then I do not wish to lay any further claim to them as my property," and 3. he expresses pessimism about the quality of what he has produced (it is only an album of halfway decent sketches) and the likelihood of its being understood (the times are too dark and one can understand the thoughts in the book properly only "by contrast with and against the background of" the thoughts in the Tractatus).
After the preface the book begins with a quotation in Latin from Augustine's Confessions. If the preface has not made us go back and read the Tractatus, and if trying to read that book has not made us stop and go back to Frege and Russell, and possibly Schopenhauer, Kant, and others too, then this Latin passage might prompt us to brush up our Latin or read Augustine. Otherwise we have to trust the narrator, who has already warned us that we are unlikely to understand what he has written.
Augustine's words are addressed to God, and they emphasize the role of the body in learning language. Bodily movements and gestures constitute a natural language, and one has to learn to use one's tongue in order to pronounce words. What drives this learning, as Augustine expresses it, is the desire to express one's will in order that one might get what one wills. There is something sinful about this: Augustine takes a rather dim view of babies and their willfulness. It is God's will we should want to be done, after all, not our own. Augustine also writes as if babies have a fully developed will (maybe not at first--when they know only to suck, to enjoy what they like, and to cry at what they dislike--but soon after this), capable of thinking and desiring a range of things. So simply watching what Latin-speakers call by what names enables one to learn Latin. Readers are probably aware that it takes more to understand the quotation from Augustine than a knowledge of the meanings of the main nouns in the passage.
There is something off about Augustine's portrayal of babies, something we might laugh at, except that it is clearly bound up with his religious beliefs. The Confessions is not a funny book. And Augustine's seemingly questionable psychology is perhaps best understood not as psychology at all but as an attempt to express our fallen nature. In this sense it sounds like the kind of thing that, at the end of the Lecture on Ethics, Wittgenstein said he would not ridicule for the life of him. So I doubt we are meant to laugh--or to keep laughing, at any rate--but we might if we fail, or refuse, to take the speaker of these words into account.
Wittgenstein's response is to say that it seems to him (so this is a personal reaction) that these words give us a certain picture of the essence of language. In this picture, he continues, we find the roots of an idea about language. This mixed metaphor might (should?) give us pause. Not to criticize Wittgenstein's writing but to think about what he might mean. What is a picture in this sense? How could it contain roots? How can he know there are these roots in this picture? Is he telling us what he thinks is going on in Augustine's mind? He doesn't say.
The next paragraph points out some things that Augustine does not mention, without any explicit criticism of him for this, and then says what Wittgenstein believes someone who describes learning language like this believes. He is thinking primarily of certain obvious nouns (the names of concrete types of objects, such as chairs) and people's names. Wittgenstein gives no reason to think that Augustine in particular is thinking this way (although it would seem reasonable enough to think this of him), and in general treats Augustine's words as if their author and original context did not matter. Perhaps that is OK, but a case could certainly be made that it isn't. Is context never important? Is Wittgenstein reading into Augustine's work ideas that he previously believed in? We don't have much basis on which to decide yet, but it seems reasonable to ask the questions.
In the next paragraph Wittgenstein changes tack (at least seemingly) and tells us to think of a use of language: Wittgenstein sends someone (who? someone he outranks? a child? a foreigner?) shopping with a piece of paper marked "five red apples." The shopkeeper does not simply hand over the apples--he looks up what colour red is and uses this information to identify which apples he should take from the drawer (!) in which he keeps them. He has memorized the number series, but still has to count up to five to know how many apples to hand over. Then Wittgenstein remarks that, "It is in this and similar ways that one operates with words." Is this claim part of what we are simply being told to imagine? Or is Wittgenstein really claiming that this story is similar to the way we operate with words? Who are "we" though? Is "operate" really what we do with words? And how similar must two things be to count as genuinely similar? Wittgenstein does not say, and there isn't really an answer to any of these questions I can think of that isn't arbitrary.
An interlocutor then asks none of these questions but one about how the shopkeeper knows what to do in response to the words "five" and "red." These would be reasonable questions if the shopkeeper were a robot and we wanted to know how he has been programmed, but otherwise they sound a bit odd to me. Wittgenstein (or the narrator) takes them in his stride, though, and brushes them aside with a mere stipulation that the shopkeeper acts as described and that "Explanations come to an end somewhere." The interlocutor then wants to know what the word "five" means (in this story, presumably, but perhaps more generally), but is told that this was not in question. The question was only how the word "five" is used.
So was it unreasonable to ask about the meaning of "five"? Does the story-teller get to dictate what the story was about? Perhaps he does, but then what is the point of the story if it does not answer the questions that other people have?
So far, then, we have been told something that Augustine wrote, told what Wittgenstein thinks in response to words like these, and told to imagine a strange story. We have not been told what we should think about anything, but have been given plenty of food for thought. Which is what Wittgenstein told us he was aiming to do in the preface.