To "catch up on" something means to do something that you missed, or that you're late on. For example, you can:
- catch up on some episodes of a TV show that you missed
- catch up on work that you couldn't do because you were on vacation for a few days
- catch up on homework that you missed because you were sick
There's a related phrase, "catch up with (someone)", which means to talk to someone who you haven't seen in a long time and share news about your lives.