To "catch up on" means to get up to date with something you missed or fell behind on. It's like when you haven't watched your favorite TV show for a while, and you need to watch the episodes you missed to know what's happening. Or if you've been away from work or school and you need to catch up on the things you missed while you were gone. So, "catch up on" is about getting back on track with things you've missed out on or fallen behind on.