It is a small tank installed between the pump and faucets--it will keep about a gallon or two of water under pressure all the time, which minimizes the number of times the pump has to come on for small amounts of water use. There is a rubber membrane inside that expands as water fills the tank--this is how the pressure is held. When the volume in the tank gets low enough, the pump kicks in to re-pressurize.
Sometimes it helps on noise and vibration also--I had one once and found no benefit to it, but others have said they work well.
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died