Echolocation is a system of location and identification used by some animals: bats, a few cetaceans (killer whales, dolphins), a few birds and shrews.
In bats this system replaces poor or non-existent vision, while in dolphins, for example, it complements the animal's vision.
Echolocation principle. © Malene Thyssen, Wikipedia, cc by sa 2.5
This system is based on the physical properties of sound waves. The animal sends waves by clicking its tongue or otherwise. These waves bounce off all obstacles found in the emitting animal's environment (see the animation above) then return to the source (ears in Chiropters), giving the animal a three-dimensional image of its environment.
Bats use echolocation to navigate. © Jan Svetlík, Flickr, cc by nc nd 2.0