All dolphins use a process called echolocation to build up a picture of their surrounding environment. This involves producing a series of audible 'clicks', which reflect off nearby objects and return to the dolphin in the form of a hologram.
Dolphins also have a vocabulary of 'clicks and whistles' uttered mostly underwater. When bow-riding, they often use these high-frequency whistles along with echolocation, to communicate or orient themselves as they jostle for the best riding position.