It's typically got a spring loaded valve on the bottom of it so it doesn't pour mead on your floor. Used while bottling. It's used as an intermediate container between the fermenter and the bottles, so you don't have to worry as much about siphoning sediment into your bottles. Something vaguely pot-like that you boil stuff in. Bigger than three gallons is good.
Stainless steel is best.
It's a pretty swell way to carbonate up 2 litres of mead or other beverage to see if you want to carbonate more of it. When I get around to finding the address of the company that makes it, I'll add a link to them here. Belonging to an order of marine mammals, including whales and dolphins. Cloth normally used to squeeze the watery stuff out of cheese curds without squeezing cheese all over the kitchen. Handy in general for filtering solids out of liquids.