What an overgrip does
An overgrip is the thin, tacky layer that wraps over the base grip your racket came with. It is the cheapest insurance in padel: a fresh overgrip stops the handle turning in your hand on a smash, soaks up sweat, and lets you build the handle up slightly to a size that suits you. It wears out — that is the point of it — so most players keep a few in the bag and change one the moment it stops feeling tacky, usually every handful of sessions.
Which one, and how it differs from a grip
Tacky overgrips like the Babolat VS Original give the stickiest hold; the Pro Tour and Dunlop Tour Pro add a touch more cushioning. If you meant the base grip under it all — the part that changes the handle itself — see grips. Wrapping one for the first time? The grips and overgrips guide walks it through.