newly planted fruit trees whose fruit is forbidden for the first three years and must be redeemed of its holiness in the fourth.
Orla is the biblical prohibition (Vayikra 19:23) against eating fruit from a tree during its first three years. Revai is the fruit of the fourth year which in earlier times would be brought to Yerushalayim and eaten there. Today, revai is normally redeemed for a nominal amount of money.
Contrary to popular belief, the years of orla restart anytime a tree is planted, whether a seed was planted or an old tree was uprooted and replanted.
Unlike terumah for example, which must be given to the kohanim, orla may not be eaten by anyone and one may also not receive benefit from it. In mixtures, orla is nullified in 1:200.
Orla is also relatively rare among agricultural laws in that it applies outside of Eretz Israel. The only difference is that in the case of doubtful orla, we assume it is orla inside Eretz Yisrael and that it is not orla outside the land.