The first portion of a harvest, offered to God as a consecration of the whole. In the OT, firstfruits acknowledged God's ownership and promised more to come. Paul applies it to Christ's resurrection (the firstfruits of those who have died) and to the Holy Spirit (the firstfruits of our future inheritance).
Christ as aparchē (1 Corinthians 15:20) means His resurrection is not an isolated miracle but the guarantee and beginning of a universal harvest — all who belong to Him will rise. The Spirit as firstfruits (Romans 8:23) means our present experience of God is real but partial — a foretaste of full redemption. The firstfruits logic: if the first is holy, the whole batch is holy (Romans 11:16).