PRP refers to Previsibines Repair Pack (PRP) Stable Branch. This mod contains 2 plugins, PPF. esm which is a bunch of bug fixes, and PRP.esp, which regenerates previsibines since PPF.esm breaks them.

Previsibines is a slang term for previs/precombines which are a couple of ways that the game improves performance.

Everything in the game is made from a mesh. Precombines takes a bunch of meshes and merges them into one, so instead of 10 meshes getting loaded, only one does, which improves performance.

Previs is another way to improve performance by not rendering items that are behind something that is currently blocking your view. Like if you stand in front of a wall, nothing behind that wall is rendered. The less that needs to be rendered, the better the performance.

If a mod changes any of the meshes that are part of precombines, then it will "break precombines". And now the game has to load those 10 meshes instead of just the one. 

The main mod breaks precombines, so I regenerate them along with previs in the previsibine patch. The PRP patches are for either v0.65.x or v0.69.x of the Previsibines Repair Pack (PRP) Stable Branch mod and the Previsibine patch are for those that don't use PRP. These patches are used in addition to the main mod.

It is highly recommended that you use one or the other.