To flag is to win on time. It's often used to describe people in blitz games who move really fast at the end of the game and get wins even with very bad positions on the board.
I guess "flagging a draw" would be if your opponent runs out of time but you can't win because there isn't enough material on the board left for you to mate, so you get a draw instead.
What does "flag" or "flagging" mean? I hear it a lot with people saying he flagged a draw or in other cases... What does this mean???