Displacement of a Moving Body
Displacement of a moving body is negative when the body is moving in the opposite direction to the reference point. This means that the body is moving away from the reference point.

Illustration
For example, if a car is moving towards the east and the reference point is to the west, then the displacement of the car would be negative.

Distance Travelled
The distance travelled by a body can never be negative. Distance is a scalar quantity and is always positive. It is the total length of the path travelled by the body, irrespective of the direction.
However, displacement is a vector quantity and can be negative when the body moves in the opposite direction to the reference point.

Explanation
Displacement takes into account the direction of motion, while distance travelled is the total length of the path covered. So, even if a body moves back and forth along a path, the distance travelled will always be positive, but the displacement can be negative if the body moves in the opposite direction to the reference point.
In summary, displacement of a moving body is negative when it moves away from the reference point, while distance travelled is always positive as it represents the total length of the path covered by the body.

Retardation.