If an object moves with constant velocity, the net force on it is zero. Constant velocity means the object's speed and direction are both unchanged, so its acceleration is zero; Newton's second law then gives a net force of zero.
This is a direct consequence of Newton's first and second laws. Newton's first law says that an object continues in uniform motion unless acted upon by a nonzero net external force. His second law expresses the relationship mathematically as F = ma, where F is net force, m is mass, and a is acceleration.
Zero net force does not mean that no forces are present. A book resting on a table has gravity pulling downward and the table's normal force pushing upward. Those forces balance. Likewise, a car moving at constant velocity may have engine thrust balanced by air resistance and rolling resistance.
A frequent mistake is to think that continued motion requires a continuing net force. Force is needed to change velocity, not to maintain constant velocity in an ideal inertial frame. If the net force were increasing or decreasing, the object's acceleration would change accordingly.