what does it mean to flag?

In chess, "to flag" is to lose on time and "flagging someone" means to beat them by winning on time. The reason for this term historically is because if you've seen the old chess clocks (before we had digital chess clocks with settings), there was a little "flag" near the 11 or 12 hours for the clock. The clocks were set before the game, so that when the clock hands moved it would lift the little "flag" and when your flag literally "fell" it meant you lost the chess game on time.

Let me see if I can google an image of an old chess clock, so you can see the little "flag" this term came from. In this photo, the little red thing is the "flag" and you can probably imagine how the hands on the clock could push the flag up and then make the "flag fall" when the hand passes it.