What is net force?

A net force (also known as a resultant force) is a vector produced when two or more forces act upon a single object. It is calculated by adding the force vectors acting upon the object. A net force can also be defined as the overall force acting on an object, when all the individual forces acting on the object are added together.

Hope this helps... :)

Fn is the vector sum of all the forces on a body. If it is not zero, then the body is accelerating at rate a = Fn/m

It's the vector sum of all the forces.

A block on an inclined plane with the wind blowing and a rocket motor on it has

gravity
friction
aerodynamic force
propulsive force from the rocket

Add 'em up to get the net force.

It is the force resulting from the addition of all forces acting on a body. An example: the net force acting on someone who is still is zero because the force of gravity is compensated by a force acting in the opposite direction by the floor, or whatever is beneath.