I have a question about why culture is ordinary, Raymond Williams mentioned that "culture is ordinary", but I really don't know what's that mean and could you please give me more example to explain it by using film ,TV, pop music,advertisement.. etc

Have you read that essay? I'm guessing this is homework. Here's a tidbit that might help explain what he is talking about (and this essay is ANCIENT! 1958) --

"In this essay, Williams takes us to his roots, his rural Welsh home, to give us an understanding of why he dislikes current (as of 1958) interpretations of culture. Williams believes that culture should be defined as both (rather than distinguished between) a whole way of life with its common meanings, as well as the processes of discovery and creativity in the arts and learning." Cultural Studies: Raymond Williams: “Culture is Ordinary”

Here's another idea on what he meant -- Literary Theory: Raymond Williams "Culture is Ordinary"

Now, figure out how U.S. culture fits into his "ordinary" theme.

I haven't read it, but I will quickly say that to me, ordinary in this context means generalized and watered down. There are so many cultures in the US that I don't even want the word culture to be described for one country in less than a big fat book.
In fact I don't want it at all. It's archaic.