When you "fill someone in", it means telling them news or information that they're missing.
For example, imagine that one of your coworkers has been out of the office on vacation for two weeks. During that time, there was a big change to one of the projects that you were working on together. So you fill your coworker in on the changes:
Rebecca, when you get a minute, can you come by my office? There are some changes that I want to fill you in on.
This expression is good for formal or professional situations.