It's their rating.
Ratings go up for a win, down for a loss. Beating a higher rated player gives more points than beating a lower rated player, etc.
Drawing a player of equal rating results in no rating change.
Your first few games have a multiplier, so at first your rating will change a lot, 100s of points for a single game, but each game after that you'll gain and lose less until you're gaining and losing a normal amount (roughly 10 points for a win vs a player rated the same as you).
Hi,
What does the number next to a player's picture mean during a game?
Thanks