As its name suggests, a countertop dishwasher is a dishwasher that can be placed on top of a counter.
This is a portable dishwasher, as opposed to a built-in one which you have to fix in one position with a permanent and dedicated water inlet.
Countertop dishwashers, on the other hand, can be fitted to any faucet with the help of a connecting adapter. The drain hose positioned behind it acts as an outlet for the dirty water.
Moreover, a countertop dishwasher consumes less energy than a built-in one. It provides some of the same benefits as a larger dishwasher. But it has a lower capacity in most cases.
This is, often a preferable option for smaller family units or singles.