The size/strength of the force
Force is a vector value - this means it has both magnitude and direction. The direction is the direction the force is applied along (so if you push a box, it would be which direction you push it in) and the magnitude is the size of the force - or how strong it is. In the box pushing example, it would be how hard you pushed the box.
Hope this helps; let me know if I can do anything else:)