The net force on an object is the vector sum of all forces acting on it at some time.
According to the principle of superposition of forces, all the forces acting on a body can be theoretically replaced with a single force that represents the vector sum of the forces:
For example, an object at rest on Earth's surface has two equal forces acting on it: the gravitational force directed toward earth, and the upward normal force exerted by the surface.
As long as the surface it is resting on is parallel to earth's surface, the normal force and the gravitational force are always equal in magnitude and opposite in direction.
A common misconception involving Newton's laws is the concept of action-reaction pair forces.
Since they are always equal in magnitude and opposite in direction, you might think they cancel out. However, you CAN NOT cancel action-reaction pair forces, because they never act on the same body: