It's an informal term which some people use to refer to a man/men.
Those dudes over there have great clothes.
What are you doing, dude?!
There was a truly dreadful American film called "Hey, Dude, Where's My Car?"
It could be replaced by "mate" in BrE, although that can be used to refer to both women and men sometimes.
I have heard the word "dude" feminised to "dudette" a couple of times, but I don't think it's very common.
Student or Learner