MAC Prep and Prime Lip is a translucent primer to be worn underneath lip colour, providing moisture to the lips.
By smoothing out your lips, this primer makes subsequent application of lipstick much easier. The end result is a more even application of lip colour, that stays on for longer than if you apply the lip colour on its own. Lip liner is no longer a necessity, as the primer helps prevent the colour feathering.
It can be used under any brand of lip colour, or you can even apply a coat on its own to soften your lips.
The primer is $33 from MAC counters or their website.