Clicking Set collisions, the following window will appear:

Main Body: lists all bodies in the current project. This is the body that starts the collisions with other bodies. For example, a pinball ball;

Collision: lists all bodies in the current project. This is the body being hit by the Main Body above;

Action: a previously made action that will trigger when the collision between the Main Body and the Collision Body happens;

Dispose on collision allows the disposal (memory and screen disposal) of the Main and/or Collided body after the collision. For example, in an asteroid game you may dispose an asteroid hit by a shoot;