What is Eye Primer?
So what is eye primer? Urban Decay Primer Potion is a silicone-based preparation product that seals the surface of your eyelids and provides a base for the eye shadow to stick to.
It's like when you paint the walls you need to put undercoat on (or primer). It seals the raw surface and covers any slight imperfections on the walls. So the paint goes on better and any oils or stains don't bleeds through. I once painted over some beams in my house without primer. After five coats I was still getting brown marks coming to the surface from the wood. It was a disaster.
Eye shadow primers firstly create a barrier from your natural eyelid oils/moisture so that eye shadow doesn't crease when they seep though.
Other eye shadow primers might use different ingredients to seal and help shadow adhere. L'Oreal for instance uses talc and beeswax.