I haven't looked myself, so I can't confirm it, but I would expect that those are indicating how that player is doing compared to other players selected in the same round. For our pools, these flags have little to no meaning.
ROUND AVERAGES (AVG): When you expand a pool team, see the numbers to the immediate left of the player’s name? That is the round that that player was drafted, or at least entered into the system by your pool admin. The AVG is calculated by taking all the players within your pool that were drafted in that round and then averaging their points. Then we add an (!) beside the player leading their respective round and an (x) beside the ones below average in their respective round.
There is a legend below the stats table in every pool that gives hints as to what everything is within the stats grid.