Ultrasound is a form of sound energy at very high frequencies that is applied to and absorbed into body tissues.
There are two basic types of application which are continuous or pulsed. Continuous ultrasound creates a thermal effect within the tissues which is thought to promote circulation. Pulsed ultrasound has other effects (but not thermal) which are thought to be biostimulatory in nature.
Ultrasound is often used in the treatment of bursitis, muscle injury/hematomas, ligament sprains etc.