How does negative work on a system occur?

The negative work is done in the opposite direction of the movement.

Explanation:

When the force applied to the body acts contrary to the movement of the body, it is considered that a negative mechanical work is performed.

In the attached image, we can see how these forces acting over one body.

In the sketch we can see how the force is moving the block to the right, it means in the direction of the movement and the friction force is acting in the opposite direction of the movement, that is the reason that the work is negative.