What are unbalanced forces?

Unbalanced forces occur when the net force on an object is not equal to zero.  When an unbalanced force occurs, a change in velocity is created.  That change in velocity might be that the object goes from not moving to moving. Alternatively, the unbalanced force may cause a moving object to speed up, slow down, or even turn.  
A simple game of tug of war can be used to illustrate balanced and unbalanced forces.  If both teams pulls on the rope in opposite directions with exactly the same size force, the forces cancel each other out.  The net force is zero.  If team "A" pulls to the right with 10,000 Newtons of force, and team "B" pulls to the left with 10,000 Newtons of force, the resulting net force is 0 Newtons.  The forces are balanced.  If one team is pulling with a greater amount of force, though, the forces are unbalanced, and the rope begins to move in the direction of the greater force.  
https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Balanced-and-Unbalanced-Forces