The idea behind liquid breathing is to allow humans to venture into the depths of the ocean--or even the cold black void of space--without the need to breathe air in gas form. Instead, explorers would wear a device that pumps oxygen-rich liquids into the lungs. This could help mitigate some of the pressure-based challenges encountered by divers in deep water and the problems associated with returning to the surface.
In the medical field, the application is already in use--premature babies born with underdeveloped lungs have used liquid breathing to stay alive until their lungs are capable of taking in air. Liquid breathing has also been used to help adults with acute respiratory failure.