Describes it pretty well. Arca is a long used system for photography, so lots of options on amazon. I bought a 12" section for like $15; plus some extra screws on a KMR handguard.
A more expensive and refined version for more money:
RRS and 419 have adapters that hook up bipods to Arca- allowing you to quickly slide the bipod back to take advantage of short supports. Bipod all the way back + bag under pistol grip = stability off short supports.
Some chassis offer Arca cuts machined into the forend. I have an XLR and MPA, both with cuts.
Geisselle just came out with a AR15 handguard with integrated Arca; as soon as they offer a large frame size, I'll buy one.