As a general term, traction means pulling on part of the body.
Most often, traction uses mechanical force (sometimes generated by weights and pulleys) to put tension on a displaced bone or joint, such as a dislocated shoulder, to put it back in position and keep it still.
Traction is also used to keep a group of muscles (such as the neck muscles) stretched to reduce muscle spasms. This is called cervical traction.
As a treatment, traction will involve a certain amount of tension to pull the body part into another position, a length of time to use the tension, and a way to keep the tension.