A keyboard has "floating" keys when the keys are elevated above the top surface of the keyboard's case.
Floating keys with an exposed sheet metal mounting plate is sometimes used by commercial manufacturers as a cheap way to provide a more luxurious appearance when in fact it is not much different from a skeleton of another keyboard that instead has a piece of plastic on top of the plate enclosing the keys. Examples of that include many keyboards from Corsair, (such as the Corsair Vengeance K90) that are often referred to as having a "metal case" by people who do not know better.
Some more expensive keyboards such as the Input Club K-Type have the top plate instead CNC-routed from a solid piece of aluminium, thus providing thick aluminium also on the keyboard's sides. This type of design is more sturdy than sheet metal and more cost-effective than if the key wells would have been excavated from a larger block.
Some keyboards with floating keys have the keys flush with edges of the keyboard's footprint primarily to omit borders around the keys to make the keyboard more compact.
Hobbyists often make custom keyboards as bolted-together sandwiches of mounting plate and bottom plate, separated by a border enclosure (or just standoffs) because that is a simple and inexpensive construction.