What does filling mean at fortnite?

When I play duo with my friend, it says fill up under what does that mean

If one of you leaves the lobby, he would provide you with a random team partner who is also looking for Duo filled up on his own.

Are you already 2. It makes no difference.

This only applies when you're alone: if you don't have anyone to play with, you can press fill up and someone will join your team.