The coming of Jesus Christ at his incarnation marked the beginning of a glorious new redemptive age with a corresponding set of blessings (the already). Yet this new age is not fully consummated and will be fulfilled in the future (the not yet). Christians today can experience a measure of the blessings and promises of heaven while still living in a fallen world that is groaning for the consummation.
The already / not yet concept is expressed in the New Testament's distinct and pronounced tension between what God has already done in fulfilling the promised of the Old Testament and what God will do yet in the future. It can be said that the already / not yet structure gives the New Testament a strong forward-looking focus.
(Adapted from A Case for Amillennialism by Kim Riddlebarger)