Just as the camera's
cullingMask determines if the camera is able to see the GameObject, the event mask determines whether the GameObject is able to receive mouse events.
Only objects visible by the camera and whose
layerMask overlaps with the camera's
eventMask will be able to receive OnMouseXXX events.
Setting this mask to zero will improve performance and is recommended if you don't use OnMouseXXX events.
See Layers for more information.
Additional resources: MonoBehaviour.OnMouseEnter, MonoBehaviour.OnMouseExit, MonoBehaviour.OnMouseOver, MonoBehaviour.OnMouseDown, MonoBehaviour.OnMouseOver, MonoBehaviour.OnMouseUp, MonoBehaviour.OnMouseDrag, MonoBehaviour.OnMouseUpAsButton.