Biomechanics can be simply the application of mechanical engineering in biology. Just as mechanics deals with solids and liquids, biomechanics deals with solid and liquid substances. The solids here are bones, muscles etc and fluids are blood and other liquids in the body. Solid biomechanics deals with orthopaedic, arthroscopic and related fields whereas fluid biomechanics deals with the effect of liquids.
Biomechanics helps in understanding the role of mechanical forces in the body and developing implants or other devices to make diagonising easy.