Concise Bible Dictionary:

The word is from πληρόω, “to fill full.” The believer is complete in Christ, or filled full, referring to all fullness dwelling in Christ: the fullness of the Godhead is in Christ, as towards the believer, and the believer, as toward God, is complete in Him (Col. 2:1010And ye are complete in him, which is the head of all principality and power: (Colossians 2:10)). The Colossians are prayed for that they might be “complete in all the will of God,” or “fully assured” in all the will of God, as most Editors read it (Col. 4:1212Epaphras, who is one of you, a servant of Christ, saluteth you, always laboring fervently for you in prayers, that ye may stand perfect and complete in all the will of God. (Colossians 4:12)).