Forced means the 2nd best move is clearly inferior.
Sometimes the 1st, 2nd, 3rd, 4th, 5th, and even more best moves are roughly equal... so it's actually worth noting when there is a big difference between the 1st and 2nd best move... these situations are called "forcing" moves.
Flagging is when you play quickly in order to win on time.
I'd like to see chess.com have a glossary of chess terms both legit and slang. Took me a while to figure out what 'gg' means for instance. Still don't know what a 'forced' move is for instance and whether if you are demarked with a 'forced' move whether I did the forcing or I got forced. Another term I've seen lately is 'flagging' . Don't know what that is. Anyway it would be especially good for newbies.