When country-sized convection cells called 'granules' cool and fall back into the interior they cause sound waves to move into the Sun in all directions. This diagram shows the path of some of the waves from one of these cells starting on the front side.

As the waves move into the Sun they get bent out back to the surface because the sound speed is higher deeper into the Sun. When they get to the surface they get reflected back into the Sun. When they get reflected, the surface layer moves and scientists are able to measure that movement.

Credits: Stanford University