Ballhawk in Madden NFL 13 means that any time you hold Y (Xbox) or Triangle (PS3) your defender will try to make the best play possible on the ball.
In Madden NFL 12, holding Y would just let you make a catch, and you still had to get into position with the left stick. Now the game will help you get to the spot of the ball as well, very similar to the defensive assist that you get by holding A (Xbox) or X (PS3).
Last year, the best way to intercept the ball was to strafe and get to the front of the catch circle as seen in our video on the EA SPORTS blog.
This season, you want to be trailing the WR; using ballhawk, you will leap up and snag the ball. Interceptions will be more prevalent than they were last season, and ballhawk should make it easier for newer players to pick passes and go to the house!