Tidal locking is the phenomenon by which a body has the same rotational period as its orbital period around a partner. So, the Moon is tidally locked to the Earth because it rotates in exactly the same time as it takes to orbit the Earth. That is why we only see one side of the Moon.

How is the Moon tidally locked to the Earth?

Gravity from Earth pulls on the closest tidal bulge, trying to keep it aligned. This creates tidal friction that slows the moon's rotation. Over time, the rotation was slowed enough that the moon's orbit and rotation matched, and the same face became tidally locked, forever pointed toward Earth.

How long has the Moon been tidal locked?

Early tides between the Earth and Moon were very powerful. As a result, the Moon became tidally “locked” within 100 million years.