The Moon and the Earth form a mechanical system called the Earth-Moon system. Both the Moon and the Earth revolve around the barycenter of the Earth-Moon system, with a period of one sidereal month. While the Moon is revolving around the Earth-Moon system, it is also rotating on its own axis, and the period of its rotation is the same as that of its revolution (the sidereal month). For this reason, the Moon always faces the Earth with the same side; people on Earth will never see the other side of the Moon.
The phenomenon of the Moon's revolution and rotation being perfectly synchronized is called "tidal locking". This kind of locking is very common among celestial bodies that orbit each other. Among Jupiter's satellites, eight are known to be tidally locked. Among Saturn's satellites, 15 are known to be tidally locked. Pluto and Charon, which are similar in size, are also tidally locked to each other.
When two celestial bodies, such as the Earth and the Moon, orbit each other, the gravitational force between them will pull the planet from a "spherical" shape to an "ellipsoidal" shape. If the deformed celestial body rotates relative to the line connecting the barycenter, the gravitational force will apply a torque to it. This torque will tend to stabilize it to a state where it no longer rotates relative to the line connecting the barycenter. This state is called tidal locking. One of the two celestial bodies, such as the Moon, will reach the locked state faster. The larger one, such as the Earth, will reach this locked state more slowly. However, under the tidal influence of the Moon, the Earth's rotation is still slowing down. If the time is long enough, it is also possible to reach the locked state. If this happens, the "day" on Earth may eventually be equivalent to a "month".