Pretty much exactly what it sounds like, water that comes in blue cans. The manufacturer claims it has a 50 year shelf life or something.

It costs several times what regular bottled water does, and it's literally just water, not flavored or sparkling or enhanced with any vitamins or electrolytes or anything, just water in a can. It's kind of a stupid thing to buy, and arguably you'd be better off just filling up some sturdy jugs from your tap and treating it with a couple drops of bleach and dumping it down the drain every so often, but I can also see the convenience of a buy-one-cry-once set-it-and-forget it prep like that.