This read-only variable returns whether or not the
instance is currently "sleeping" (true) or not
(false), A "sleeping" instance is one that is not actively
engaged in any physical simulation. GameMaker: Studio will
put objects to sleep to save simulation cycles when an instance is
at rest and not in collision with another instance.
The above code checks to see if the object is being actively simulated or not and if it is not it is destroyed.