What phenomenon results in the Moon's rotation period slowing until it matches its orbital period around Earth?

Synchronous rotation or tidal locking

Synchronous rotation, also known as tidal locking, is a direct consequence of the long-term gravitational interaction between the Earth and the Moon. Over vast periods, the Earth's gravity exerted a tidal force on the Moon, causing its rotation speed to gradually decrease. This process continued until the Moon's rotation rate perfectly matched its orbital period around the Earth. Consequently, the Moon now completes exactly one spin on its axis for every single orbit it makes around our planet, resulting in one hemisphere—the near side—being perpetually oriented toward Earth, while the other hemisphere—the far side—is always facing away.