ערלה

One of the Mitzvot mentioned in קדושים is the Mitzva of ערלה. This refers to the prohibition of eating fruits of a tree during the first three years of their growth.

The interesting aspect of this commandment, is that it is described as הלכה למשה מסיני, given by Hashem to Moshe at Sinai. It seems to be a very clear cut Mitzva.

The answer is that there is a difference between Israel and the Diaspora. In Israel, this law must be followed meticulously. This is why wineries have a Mashgiach on the premises, to be certain there is no ערלה violation.

Outside of Israel, tradition teaches that only known ערלה is forbidden. Therefore, when buying fruit there, one need not ask if it is ערלה fruit or not. It is only when it is known to be definite ערלה, that it is also forbidden in חוץ לארץ. This is what is meant by הלכה למשה מסיני.