If I remember anything about Italian, "sei" is for singular "you", "siete" is for plural "you". In old English there was "thou" for singular "you", maybe this will help to understand.
SEI is for the second SINGULAR person, SIETE is for the second PLURAL person. TU sei , VOI siete . Tu sei veloce. Voi siete veloci. ---->You are fast . You are fast.