My friends consider me an encyclopedia of hideous internet fetishes, so maybe for once I can use that power to explain some literal bullshit. So for anyone curious, read on because I'm gonna break down vore.
Okay, vore as a concept is actually pretty simple, but you've got several major factors at play. First off, let's make the less than obvious connection that it's basically like a blowjob for your whole body on one end, and a full body fucking on the other. Read a bit of their dialogue and put it out of your mind that these characters are eating one another. They'll talk about how tight it is, or the slipperyness, or something else that could basically be used to describe sex in general. It's dumb, but just imagine your whole body is an erogenous zone basically.
Then you get into the control factor. Standard BDSM power dynamic rules apply. One person is fully submitting themselves to another. The reasons for this being sexy are an entire beast of their own, and there's no shortage of decent explanations on this front, so I'll leave it there.
You're also dealing with various fixations. For instance an oral fixation, belly fixation, etc. Just some particular part of human anatomy styled in a particular way that they enjoy. Why do some people like pregnant or morbidly obese women? Once again, no shortage of answers.
Then you reach the fantasy elements of it all. Vore has a variety of different "rules" so to speak. Some people like the idea of just hanging out inside someone. Some people like the idea of being melted into goop, and fusing with their "predator" and becoming their sentient ass or something. These would each require their own breakdowns, but it's safe to say that the general idea is that someone is having a good time, and they beat their meat to that idea. Mix a little masochism in there if you don't understand the mindset of someone who wants to be painfully dissolved.
It all links back up to fetishes higher on the iceberg, really.