Collision filtering is a system for preventing collision between shapes. For example, say you make a character that rides a bicycle. You want the bicycle to collide with the terrain and the character to collide with the terrain, but you don't want the character to collide with the bicycle (because they must overlap). Box2D supports such collision filtering using categories and groups.
Read more about this in the Box2D documentation.categoryBits | number | A 16-bit integer representing category membership. |
maskBits | number | A 16-bit integer representing masked categories. |
groupIndex | number | An integer representing the group index. |