The Teen Titans is a superhero team consisting of young superheroes, including Robin.
The team was first mentioned by Robin during "Finding Mr. Right" when he opened up to Batman about how everyone in the Teen Titans was getting a nemesis except for him. The team was mentioned again by Queen of Fables in "The Line" when she contrasted her own morals against the team, stating that unlike the Teen Titans she did not have "a line".
In "There's No Ivy in Team", Batgirl mentions that Nightwing was a Teen Titan although it is unclear if he is still on the team or if that was in the past. However, considering Robin is part of the team and Nightwing didn't even know how old he was, it can be presumed he is not currently a member.