It means give someone the information that they are missing. 'I haven't been here for a few days. Can you fill me in on what's been happening around here?' This means tell me what has happened during the time that I was away - fill in the gaps in my knowledge.

If you "fill somebody in," it means you are telling them all about the situation or concept. You are informing him or her. "Filling in" any gaps in their knowledge.

Hi there, "to fill somebody in" means to let them know what is happening. For example: A: I had a terrible day today. B: Oh no, what happened? A: It's a long story, I'll fill you in when I see you! You are filling in the gaps of the other person's knowledge :) Hope that helps!