F stands for Fail while L stands for Loss.
It's kind of a slang to say L to people who lose at something, most notably derived from game scoring at sports.
6ix9ine went to jail.
"Hold this L."
It's also slang to say "F" to people who lose at something. It's from "press F to pay respects" in a game but I'm sure which game.
So the same logic also says we should keep Fs