Constant motion means that the velocity of the object does not change.
The speed of an object changes when force is applied. Therefore, for an object with constant velocity, the sum of all the forces applied is equal to zero.
For example, suppose a car runs at a constant speed. When you add up all the forces on this car (called 'net force'), you get '0'. This is due to the friction of the car being offset by the power of the engine.