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. 2:10). 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. 4:12).