Collision Map is a special operator that allows create contact or "collision" maps where objects or particles contact one specific object.
The collision map only records the map for only one object.
The first object in the list is the object whose map will calculated, the next objects and the group particle are the collision objects to scan.
The basic idea behind the collision map, you have a ground plane where some per hand animated objects collide, so you want the collision area to use as fragment trigger map in the Fragment operator to fragment the ground on this areas.