In the Bible "firstfruits" refers to the earliest part of a harvest, which Israel is commanded to bring to the LORD as an offering, described in Leviticus 23.
Leviticus 23:10 in the King James Version reads:
Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest:
The verse appears in Leviticus 23, in the laws for the annual feasts. Read it in context at Leviticus 23:10, or the full chapter at Leviticus 23. Encyclopedia entry: en.wikipedia.org/wiki/First_Fruits.