Orthotics are designed to fit inside your shoes to help increase mobility within the foot which has restricted movement due to a
biomechanical dysfunction (functional). Alternatively they help protect the foot against excessive force building up corns or callus (accommodative). In a nut shell, they'll help your foot move better or protect them against increased pressure in the wrong area.
Orthotics work by positioning the foot into a posture that aids the foot to move in the most efficient movement to ensure a smooth gait / walking pattern, or to deflect pressure away from an area of pain / trauma.