The sum of all the forces acting on a body is the resultant force or net force on the body.
A body is often subject to multiple forces pointing in different directions.
These forces, like all vectors, can be combined by adding them up to give the resultant force.
Remember because they are vectors, the direction of the forces needs to be taken into account when they are added.
The resultant force is a single force that has the same effect on the body as all of the individual forces acting together.
In the two diagrams below, different forces $$A$$, $$B$$ and $$C$$ act on a body. They can be added together to give the resultant force $$R.$$