Libration
The slow rocking that reveals more than half the Moon's surface over time.
The Moon keeps one face toward Earth, but not rigidly. Its rotation is uniform while its orbital speed is not, so the visible face appears to rock east and west by up to 8°; the tilt of its axis adds a north–south nod of about 7°.
Together these bring roughly 59% of the lunar surface into view at one time or another, rather than the 50% a perfectly locked Moon would show. The remaining 41% was unseen until Luna 3 photographed the far side in 1959.