Biomechanics is simply the term for applying mechanical laws to living things.
A qualified physiotherapist can give you a full biomechanical assessment, which should diagnose any conditions related to faulty biomechanics, for example over-pronated feet, or tight muscles or ligaments. These can inhibit proper body function and give rise to a whole host of further musculoskeletal problems e.g. ankle and foot pain (including achilles tendon issues) and Plantar fasciitis.
Common knee issues such as anterior knee pain are thought to have links with poor mechanical function, and sufferers of spinal pain should always have a functional assessment to assess adverse mechanical stresses.
Once a problem is diagnosed, an appropriate course of treatment can be mapped, to achieve recovery of normal function.