Sloth and torpor is an unpleasant state of body and mind, too stiff to leap into the bliss of Jhana and too blinded to spot any insights. In short, it is a complete waste of precious time. Restlessness refers to a mind which is like a monkey, always swinging on to the next branch, never able to stay long with anything. It is caused by the fault-finding state of mind which cannot be satisfied with things as they are, and so has to move on to the promise of something better, forever just beyond. The Lord Buddha compared restlessness to being a slave, continually having to jump to the orders of a tyrannical boss who always demands perfection and so never lets one stop.
Restlessness is overcome by developing contentment, which is the opposite of fault-finding.
One learns the simple joy of being satisfied with little, rather than always wanting more. One is grateful for this moment, rather than picking out its deficiencies. For instance, in meditation restlessness is often the impatience to move quickly on to the next stage. The fastest progress, though is achieved by those who are content with the stage they are on now. It is the deepening of that contentment that ripens into the next stage.