Axial play means the looseness between balls and raceways in a ball bearing. To be precise it is the maximum displacement between two rings of a ball bearing .The direction of motion is taken parallel to the bearing axis. The bearing is taken to be in the un-mounted state. The amount of this play depends upon manufacturing methods. Actually, what happens is that when a bearing runs under a load, force gets transmitted from one bearing ring to another. It is transmitted through the balls. Now, this force or stress can be extremely high since the area between the rings and the balls is extremely small. This stress has a lot of effect on the life of the bearing and its functioning. So, it is extremely necessary to choose the right type of bearing for a particular load.