Why the moon always shows the same face

The Moon shows one face because it is tidally locked, rotating once in the same time it takes to orbit the Earth. The spin keeps the same hemisphere pointed inward, which is why the familiar dark patches do not rotate out of view.

The Moon is not frozen in space with no rotation of its own, because if it did not turn relative to the stars every side would swing past the Earth during a month. The matched spin is what hides the far side, not a failure to rotate.

The mechanism is more particular than the slogan. Earth's gravity raises a slight bulge on the Moon, and if the spin and the orbit disagree that bulge is dragged out of line. The pull on the offset bulge torques the spin, and over a long time the torque slowed the Moon until the bulge could stay aimed at Earth.

Follow it one step further. Locked does not mean the view is perfectly still, because the orbit is elliptical, so the orbital speed varies while the spin stays nearly steady, and the axis is slightly tipped. Those mismatches let strips of the far side peek in and out, which is libration.

The case worth keeping in mind is specific. A crater you pick out with binoculars is still in the same part of the disk next month and next year. The far side, mapped by spacecraft, is a different landscape that never faces the ground, so the steadiness of the naked-eye face is the lock rather than a painted mask.

That case has a boundary. The lock matches the spin period to the orbital period on average, while libration means observers eventually see a bit more than half the surface, in slivers around the limb. Waiting in one place on Earth still never brings the deep far side into view.