NOTE: this is a testbed for a Unity feature that has not shipped yet! The project will not work with any public Unity version, and things in it might and will be broken.
"Scriptable Render Loops" is a potential future Unity feature, think "Command Buffers, take two". We plan to ship the feature, and a new modern built-in rendering loop with it. For now you can look around if you're really curious, but like said above, this is not useful for any public Unity version yet.
There's a more detailed overview document here: ScriptableRenderLoop google doc
Did we mention it's a very WIP, no promises, may or might not ship feature, anything and everything in it can change? It totally is.