Dolphins are able to send out sound beams from their foreheads that bounce back to them and give them information about the objects around them. These sound beams are called sonar. Dolphins use sonar so that they will know what is in their surroundings.
Instead of a keen sense of sight they have sonar instead. Sonar can be just as good as having excellent vision. Actually sonar is so good that a dolphin can tell a golf ball from a ping pong call just based on the differing densities of the two. This is also known as echolocation because the echo of sound is what helps dolphins identify the things around them.