It's basically the concept, being able to take someone inside of you or be inside of someone. The idea and "feel" of holding someone/being held inside of someone is stimulating to the mindset for some people. It can also be derived from the rush of adrenaline it gives some people.
vore
The following is a list of role play terms that are commonly found everywhere in the vore community.
Vore — A tricky word that includes too much to be easily specified; a group-name for most of the fetishes displayed on this site.
One could say that it originates from consuming someone whole, but consumption usually means eating, and eating involves digestion, which isn't necessary when some speak of vore.
What is a "Vorarephile" ?