Presumably this is talking about the low-tech practice where a homework assignment is a physical document on which the teacher has put all the exercises to be done and all the problems to be solved. If your friend was absent, he did not have a chance to pick up the document, and so would have nothing to hand in at the next class. You would be a good friend if you collected an extra copy, for your absent friend. It sounds like an old movie, doesn't it? Many schools now post assignments on an intranet.

It means "collect", in this case from the teacher or the school.