What it means
A roster is the line-up of romantic or flirty prospects someone keeps on the go all at once, like a sports team's list of players. You stay on the roster until you make the starting eleven or quietly get dropped.
Usage examples
"He is not serious about anyone, he just keeps a whole roster going."
"She trimmed her roster down to one once she actually caught feelings."
"He's got a whole roster and still acts shocked when one of them clocks what's going on."
"I thought I was special, then I realised I was just on her roster with three other guys."
Where it comes from
It comes straight from sports talk, where a roster is the official list of players on a team. In dating slang, people lifted it for someone’s rotation of flirty options or people they’re casually seeing, with all the benching, cutting, and starting-lineup jokes baked in.
