Attending the flag is when someone stands by the flag when a golfer is putting it so they can see it from far distances. Once the golfer hits, the person removes the flag so the ball does not hit it.
Could you attend the flag for me? I can't quite see the hole from this distance.
This used to be more common because while on the green, it used to be a penalty if your ball hit the flag. As of 2019, to speed up the pace of play, the USGA made it legal to leave the flag in while putting, so "attending the flag" is not used as much anymore.