The Definition: Tikkun is a term used in Rabbinic Judaism to describe the ongoing process of repairing and perfecting the world.
In the Book of Genesis, it states in verse 2:2, “And on the seventh day God ended His work which He had done, and He rested on the seventh day from all His work which He had done.” The phrase “rested on the seventh day” isn’t meant as a need for God to repair himself, but rather as a model for human behavior and an expression of God’s satisfaction with the work He had done.
The Tikkun interpretation of God’s rest at the end of creation (Genesis 2:2) suggests that the end of creation is the start of humanity’s relationship with God as we work together to perfect and preserve the world.
God’s rest is an example for us to take a break from our work, reflect on the beauty of creation and to steward and care for the world as we work toward Tikkun.
Tikkun doesn’t mean that God had to "repair" himself, but rather it is God’s call for humanity to take part in the process of making the world better, in relationship with God. So, while God’s work of creation is “complete,” humanity’s role is to add to the eternal "flourishing" and renewal of creation, fulfilling the idea of Tikkun.
Do all Christians agree with this interpretation? That's another whole story.