Note that this statement is not implying that one can not think of a world (or universe, etc) where they do not exist; rather, it is saying that when someone thinks of a world where they don’t exist, they place themselves as an external observer of this world and hence they still exist in this world but in a state that does not interact with it.  This is the inherent problem with attempting to contemplate your own nonexistence—you will always be imagining it as an external observer and therefore still exist.  Summarizing, when contemplating your own nonexistence, you do not place yourself in a state of not existing but merely regulate yourself to an extant external observer witnessing events but never influencing them.

This impossibility to conceive of nonexistence leads directly into the following

It is impossible to truly understand death

Since death is a state of not existing and the mind can never conceive of its own nonexistence, it is impossible to conceive what it will be like to be dead.