The moon takes around 28 days to orbit the Earth, during which time it rotates once. If you imagine the system from above you will see that if it only rotates once during its orbit, it will always have the same face pointing at the Earth.

You might imagine it using the hands of a clock. If you stick a coin on the tip of the minute hand, as the hand moves around the clock the same part of the coin is always pointing towards the centre of the clock-face. After the minute hand has been around the clock once, the coin will have rotated once.