Do you see double?
We all are having a binocular vision where images arise from both the eye optically. and fuse to form one image because of their synchronized action in brain. When one of the eyes do not move in conjunction in coordinated synchronized manner then instead of one there are two images formed called double vision or diplopia. This occurs when the nerve supplying the movement of eyeballs lose their proper function, as happens in various ocular neuropathies ( involvement of cranial nerves controlling movement of eye balls).
Similar imbalance can occur when the muscles of the eyeball movement get fatigued on continuous functioning as occurs in myasthenia gravis which regain function after resting or medication.