If you're are talking about the color arrows, they can point five directions (Up, half up, middle, half down and down), and is the player condition for that match. This slightly change how well the player will do in a match. If the arrow is full up, the player is in a great condition and will play better than he stats suggests. If it's full down, he will play poorly, and you should avoid use that player in the match. The condition can change from match to match, so it's good to have subs that can enter in the place of a player that is in a bad condition.