Varicose veins are veins that have become enlarged and twisted. The term commonly refers to the veins on the leg, although varicose veinscan occur elsewhere. Veins have pairs of leaflet valves to prevent blood from flowing backwards (retrograde flow or venous reflux).are veins that have become enlarged and twisted. The term commonly refers to the veins on the leg, although varicose veinscan occur elsewhere. Veins have pairs of leaflet valves to prevent blood from flowing backwards (retrograde flow or venous reflux).