Here's why:

* Oscillation: An oscillation is a repetitive back-and-forth motion. Think of a swinging pendulum or a bouncing spring.

* Displacement: Displacement is the overall change in position from the starting point to the ending point. It's a vector quantity, meaning it has both magnitude (how much) and direction.

* One Oscillation: In one complete oscillation, an object starts at a point, moves to an extreme position, then back to the starting point.

Since the object ends up at the same position it started, its net displacement is zero.

Important Note: While the displacement is zero, the distance traveled during one oscillation is not zero. The object covers a certain distance as it moves back and forth.