Numbers 18:13
And whatsoever is first ripe in the land, which they shall bring unto the LORD, shall be thine; every one that is clean in thine house shall eat of it. And whatsoever is first ripe in the land, which they shall bring unto the LORD, shall be thine; every one that is clean in thine house shall eat of it.
Whatever ripened first in the land and was brought to the Lord became the portion of the priests. It was set apart for God and then devoted to their use in connection with the sanctuary. This setting apart of the first fruits taught Israel to put God first. It kept them mindful that the land and its harvest belonged to Him.