net force is the total amount of things working on an object. if your acceleration is 0, the net force is 0 because the amount you have doing work on the object is the exact same amount acting against it ie. the force of a guy pushing a box at a constant speed is going to equal the amount of frictional force in the opposite direction.
if the thing starts to accelerate, it is because the forces acting on the object are imbalanced, causing its velocity to change.
the guy will start pushing harder on the box to speed it up, he needs to push harder than the amount of frictional force going in the opposite direction hes pushing.
maybe disregard that last examlple it makes it more confusing