Tidal locking, in the best-known case, occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. This is known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner.

What does it mean for a planet to be tidally locked?

A tidally-locked planet in its orbit around a star keeps the same face towards the star. ... The rotation and revolution periods, even if initially different, could get synchronized over time due to tides on the planet because of the gravitational effect of the star, hence the term tidal-locking.

How did the moon get tidally locked?

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.