The English word "firstfruit" appears only once in the King James translation's Old Testament. It comes from the Hebrew word reshiyth (Strong's Concordance #H7225) used to designate the first fruits of the harvest.
The firstfruit also of thy corn, of thy wine, and of thine oil, and the first of the fleece of thy sheep, shalt thou give him (Deuteronomy 18:4).
The plural of this word, however, is found twenty-five times in the Old Testament. It is derived from the Hebrew reshiyth (mentioned above) or from the word bikkur (Strong's #H1061).
The underlying principle of firstfruits was that, individually, Israelites were to offer God the first of what they harvested in gratitude of his abundant blessings. These offerings made first at the wilderness tabernacle and then at the temple, were then distributed to the Levitical priests.
The tribe of Levi was given the firstfruits of the harvest (as well as tithes, other offerings, etc.) in support of their dedicated service to God and as an inheritance from Him. They, unlike the other Israelite tribes, were not allocated area in the Promised Land they could individually own and cultivate (Numbers 18:24, Joshua 14:3 - 4, 18:7, etc.).
The phrase "first of the firstfruits" appears only in Exodus 23:19, 34:26 and Ezekiel 44:30. Its use underscores the principle that offerings to God during the three annual Feast day periods (Passover, Pentecost and Tabernacles) were to be the first and choicest of the harvest.
New Testament meaning
In the KJV New Testament, the Greek word aparche (Strong's G536) is translated both as "firstfruit" and its plural in Romans 8:23, 11:16, 16:5, 1Corinthians 15:20, 23, 16:15, James 1:18 and Revelation 14:4.
Of his own will begat he us with the word of truth, that we should be a kind of firstfruits of his creatures (James 1:18).
The term "firstfruits" was used to refer to those who first became Christians in a particular geographic area (Romans 16:5, 1Corinthians 16:15). The Apostle Paul also used the term to denote that Jesus was the first of many other faithful servants God will bring back to life from the dead (1Corinthians 15:20, 23).
Paul labels the select few or remnant of Israel, called to be Christians, as the firstfruits of countless other brethren who will also, someday, be called and converted (Romans 11:16). This, of course, implies that many others will be converted at a time chosen by God.
Lastly, the conversion of 144,000 Israelites (excluding the tribe of Dan), during the early stages of the Day of the Lord, is called the firstfruits of God and the Lamb (Revelation 14:4).