Prophylaxis aims to prevent the improvement of the opponent.
It prevents also the attack oppportunity of the opponent.
Black prevents the move of the White Knight , Nb5 , by playing a6. Thanks to the pawn White cannot use b5-square as an outpost in the attack.
Another example of Prophylaxis : Bishop pinned the c3-Knight. and prevented the Knight move.
After Re8 , Rook attacks the King and checks . You can block this check by moving the Bishop to e8.
1. Re8 Be8
This is called as block-check.