Due to lunar libration, an observer on Earth can see 59% of the Moon's surface.
Although the Moon is locked in orbit with the Earth, so that we only see one face from Earth, that face seems to wobble somewhat. This is called libration.
Libration of latitude occurs because the Moon's axis of rotation is not perpendicular to the plane of its orbit, so an observer in the northern hemisphere can sometimes see over the Moon's north pole and under its south pole.
Libration of longitude is caused by the eccentricity of the Moon's orbit, so an observer can sometimes see over the eastern and western edges of the Moon.
Both of these phenomena are cyclic over the course of one lunar cycle.