With the mentality arrows you get your players forward or playing more defensively. For instance, when the player has been set with the arrow to be of assistance of the defense he will utilize his defense skills to protect the goalkeeper's net and more of his actions should take place in the defense. On the other hand when the player has been set to be part of his attack he should be looking to support your attacking line.
Please have in mind that there are no precise amounts of space that the player is forced to go backwards or forwards. The arrows designate the function that the player will have to play on the pitch.
To set the Arrows, select the Player Mentality button in the Squad screen.