Wittgenstein begins the lecture with an odd kind of definition of his key term. He says he is going to use Moore's "explanation" of ethics as "the general enquiry into what is good," but then immediately says he is going to use 'Ethics' in a wider sense than this. He says that the clearest possible understanding of what he means by 'Ethics' can be got by his giving a collection of "more or less synonymous expressions," so that an overall impression will be created, similar to the kind of impression Galton produced when he made a composite photograph. Galton's technique is dodgy in two ways: it isn't reliable (the results can be manipulated by choosing particular people to include or exclude, for instance, and his attempts to reveal a facial type of this or that criminal (e.g. murderer, thief, etc.) were apparently unsuccessful), and it is connected with racism. Galton considered the Chinese to be racially superior to Africans, for instance, and the point of his technique is to produce a kind of literal racial stereotype. Nevertheless, it seems that the technique can be effective: see here, for instance. And the idea of family resemblance seems generally harmless. (Wittgenstein's thinking here is reminiscent of the disagreement between Berkeley and Locke on abstract ideas. Does Galton disprove Berkeley's claim that "nothing abstract or general can be made really to exist"?)
Ethics, Wittgenstein says, has to do with absolute value, absolute goodness, absolute importance, and so on, as distinct from merely relative (means-end) value, etc. Since this is not a matter of fact (it is not a scientific fact that this or that has absolute value or is absolutely good), it is a matter of feeling. So Wittgenstein talks about experiences he has that make him want to use such expressions as "absolute value" and so on. There is a direct link between the experience or feeling and the expression (form of words) used to express it. The experience is what is expressed by or in the expression. So although we're talking about psychology, we are also talking about grammar.
Wittgenstein has one experience that he associates above all with ethics as he means it, namely the feeling of wonder that the world should exist at all. But he has a couple of others too: the feeling of being absolutely safe (which reminds me of Socrates' saying that a good person cannot be harmed--that being poisoned to death is somehow not really suffering any harm) and the feeling of guilt.
Each of these experiences also has a religious form of expression, he suggests:
For the first of them is, I believe, exactly what people were referring to when they said that God had created the world; and the experience of absolute safety has been described by saying that we feel safe in the hands of God. A third experience of the same kind is that of feeling guilty and again this was described by the phrase that God disapproves of our conduct.But if the form of expression is inseparable from what is expressed, as I suggested, then how can a religious and a non-religious form of words refer to exactly the same thing? I think we have to leave open the possibility that when the words are different the experience is different. But just as different definitions of 'ethics' can share a family resemblance, so too can different expressions of ethical or religious experiences. In that case, having 'religious' experiences does not commit one to being religious--the experience can be translated into different language.
Religion offers a way to tie these experiences together, and to other beliefs, feelings, etc., in away that might help you make sense of the world, of life. But it also ties you, or so it seems, to certain texts, forms of words, rituals, and so on, that might seem tired or incredible. If the 'metaphor' used to express these real experiences has become dead, then there is reason to reject it. Then the challenge becomes how to talk or think about these experiences, and how to connect them with the rest of your life. This is a challenge we cannot meet, according to Wittgenstein (and never could, even within religion): "the tendency of all men who ever tried to write or talk Ethics or Religion was to run against the boundaries of language. This running against the walls of our cage is perfectly, absolutely hopeless." But unless we just don't think about these supremely valuable experiences (assuming we have them, or have had them), then we have no option but to at least think nonsense. Our lives, at least on the inside, will then consist in part of a hopeless running against the walls of language. And what is this if not a form of creative expression, of art?