If the net force on the object is zero
This means that there are either no forces acting on the object, or (more likely) that the forces acting are counteracting each other, both in size and direction.
If I'm standing on a floor, gravity is pulling me down. My weight pushes in the floor. The spring power of the floor is pushing me up.
If the floor is strong enough, these two powers will be in exact equilibrium, so there's no net force acting upon me.
(if the floor isn't strong enough, the force of gravity will prevail, the floor will break, and I will be accererated...).
This part of physics (where forces are in equilibrium) is called statics.