net force

Real objects feel many forces at once: gravity down, the floor up, a push sideways, friction backward. The net force is what you get when you add them all together, taking direction into account. It is the single force that would have the same effect as all of them combined. The picture is a tug-of-war, where what matters is which side pulls harder, and by how much.

The net force is the vector sum of every force acting on an object: F_net = F_1 + F_2 + F_3 + ..., added as vectors (usually by breaking each force into x and y components and summing those separately). Only the net force determines the acceleration, through Newton's second law F_net = m a. If the forces balance so their sum is zero, F_net = 0, the object does not accelerate.

This is why a book on a table stays still even though gravity pulls it down: the table's upward normal force exactly cancels gravity, so the net force is zero. And in a tug-of-war the rope accelerates toward whichever team's pull leaves a nonzero net force.