The plantar fascia is the thick connective tissue (aponeurosis) which supports the arch on the bottom (plantar side) of the foot. It runs from the tuberosity of the calcaneus (heel bone) forward to the heads of the metatarsal bones (the bone between each toe and the bones of the mid-foot). The plantar fascia is a broad structure that spans between the medial calcaneal tubercle and the proximal phalanges of the toes. Recent studies suggest that the plantar fascia is actually an aponeurosis rather than true fascia.

Source: Wikipedia