Here's why:

* The Moon doesn't produce its own light: It reflects sunlight.

* The terminator is the dividing line: It separates the sunlit half of the Moon from the shadowed half.

* It's constantly changing: As the Moon orbits Earth, the position of the terminator shifts, making the Moon appear to go through phases.

Think of it like the line between day and night on Earth! The terminator is like the "sunrise" or "sunset" line on the Moon.