The seventh century imperial church had a political problem it was trying to solve with a doctrinal formula. Egypt, Syria and Armenia had rejected Chalcedon two centuries earlier and were now largely outside imperial control; the emperors wanted them back, and successive attempts were made to find a statement about Christ that both sides could sign.
Monothelitism was the last of these attempts. It conceded the two natures Chalcedon had defined and proposed that there is nonetheless only one will in Christ, so that the unity everyone wanted to affirm could be located in the will rather than in the nature. It had real theological support and imperial backing, and for a time it looked like it might work.
The council rejected it, and the reason it gave is worth stating because it is not obvious. A will belongs to a nature, not to a person: willing is something a human being does as a human being. A Christ with only one will is therefore a Christ who is not fully human in the way that matters most, since the human capacity to choose has been absorbed. The Gethsemane scene, in which Christ prays that the cup pass from him and then wills otherwise, was read as showing two wills in operation and one of them freely conforming to the other.
Two natural wills and two natural operations were defined, and monothelitism was condemned by name along with those who had taught it. Nothing further about the person of Christ has been defined by a council both East and West receive.