When the forces acting upon an object are balanced, the object will maintain its state of motion; that is, its velocity remains constant. If the forces are unbalanced, the object will either speed up or slow down. The reverse logic is also true: if an object has a constant velocity, then the forces are balanced. And if an object is speeding up or slowing down, the forces are unbalanced.
 

A dot diagram shows how an object is moving by representing its location at consecutive second (or some time interval) by a dot. By examining the dots over the course of time, one can tell if the object is speeding up, slowing down or moving with a constant speed.

If the spacing between dots is becoming less and less over the course of time, then the object is slowing down.  And if the spacing between dots is becoming greater and greater over the course of time, then the object is speeding up. In both instances, there is an unbalanced force acting upon the object. 

Finally, it is important that you intepret a dot diagram by reading it in the appropriate direction - either from left to right or from right to left. If an object is moving rightward, then the earliest dots are on the right side of the diagram and you should "read the diagram" from left to right. On the other hand, it an object is moving leftward, then the earliest dots are on the right side of the diagram; thus you should "read the diagram" from right to left.