Glorification is the final, perfect communion in glory that the members of the invisible church have with Christ, perfected at the resurrection and day of judgment. It is the end of all the benefits believers receive by the death of Christ, wherein God fully manifests His glory and bestows eternal blessedness upon His people.
In the Reformed tradition, glorification represents the culmination of our union with Christ. As the Westminster Larger Catechism — Question 82 teaches, the communion in glory that the members of the invisible church have with Christ begins in this life, continues immediately after death, and is finally perfected at the resurrection and day of judgment. This is the moment when the work of grace is complete, and believers are fully brought into the eternal fruition of the blessed God.
The ultimate purpose of our redemption and glorification is the manifestation of God's own glory. As stated in the 1689 LBCF Ch.3 §3, God predestinated some to eternal life specifically for the praise of His glorious grace. John Owen notes in The Death of Death in the Death of Christ that the end of Christ's death is our recovery from the state of misery into eternal communion with God. While we are currently in a state of growing in holiness, we look forward to the state of glory, where, as the 1689 LBCF Ch.9 §5 explains, the will of man is made perfectly and immutably free to good alone.