“For three years it shall be for you forbidden; it may not be eaten!” (Vayikra 19:23)
It is a negative commandment that one may not eat fruit of a tree during the time of its prohibition, which is for the first three years after its being planted, whether it was planted as new, or if he planted a branch from a tree, as it says “For three years it shall be for you forbidden; it may not be eaten!”.
Amongst the roots of this commandment are that Hashem wanted a person to be aroused to praise Him at the beginning of the best of the fruit of his trees, so that the pleasantness and blessing of Hashem will be bestowed upon him, and his fruit will thereby be blessed, and the best of the fruit start growing in the fourth year, and He therefore forbade the fruit for the first three years. The Ramban writes a similar reason, and adds that the fruit of a tree at the beginning of its growth damages the body and is bad to eat.
This commandment applies in all places and at all times, to men and to women. Moshe was told at Mount Sinai that this commandment applies also outside Eretz Yisrael, however, in that self-same “Halachah LeMoshe MiSinai” it says further that in those areas the prohibition applies only to those fruit which we know are definitely from the first three years of a tree. If there is any doubt in the matter, one may eat it. (However, fruit from Eretz Yisrael are forbidden even if one is unsure if they are Orlah, as with all uncertainties about a Biblical prohibition when one must be stringent if one is unsure whether it is permitted.)