can someone explain this sentence for me? I'm not english.
"he had taken in the opening act for one of his band's gigs-the Sex Pistols"
from wikipedia: http://en.wikipedia.org/wiki/The_clash
"take in" is another, somewhat informal way of saying "watch" when talking about performances, movies, concerts, etc.
Do you want to take in a movie?
When we get to New York, we'll have a nice dinner, take in a show and walk through Central Park.
Note that this is used very casually, usually in the planning stages, and doesn't replace "watch" in every situation. For example, you wouldn't normally ask someone: "Have you taken in Lord of the Rings?" when you mean to ask if they've ever seen that movie. You would ask like that, for example, if it were actually showing at the time and you're wondering if they've gotten around to seeing it.
In your sentence, the writer is mentioning sitting through this performance in an offhanded casual way, perhaps to show that even through they didn't know the band at the time, they would later become world famous.