Essentially it's a base kit/template that you would buy from the asset store to help you with development time.
It also really helps artist (Like myself) to create games even if they don't know how to code.
Rfps (realistic first person shooter kit) is just a really great tool if built on top of to make a good single player fps game.
Sadly the kit has been outdated for a while so it's impossible to buy or use in newer game engines unless you know how to convert it.