When something envelops you, it wraps around you. The Snuggie Blanket is so popular because it envelops your body, leaving only your head uncovered. Sure, it looks dorky, but it keeps you extra warm.
You probably noticed that envelop looks just like the noun envelope, minus that last letter "e", and that's an easy way to help you remember its meaning, too. Picture what an envelope does — it seals a letter completely inside, or envelops it. Envelop can also be used in a symbolic way, to describe something that consumes or occupies your mind. If the pretty girl you just met starts to envelop your thoughts, your work will suffer because you can't concentrate. Envelop comes from the late 14c. word envolupen, meaning "be involved in."