Take a [second] tithe of all the seed crops that come forth in the field each year.
Aser te'aser et kol-tvu'at zar'echa hayotse hasadeh shanah shanah.
(Rashi; Yad, Maaser Sheni 1:1). This was in addition to the first tithe given to the Levites (Numbers 18:24). It was given in all years of the Sabbatical cycle except the third and sixth (see Deuteronomy 14:28).
(see Yerushalmi, Maaser Sheni 1:1; Yad, Terumoth 2:6; HaKethav VeHaKabbalah; cf. Ramban).