#### Answer

A slope of $0$ means the line is horizontal.

#### Work Step by Step

RECALL:
Slope is equal to the rise (change in y) over run (change in x).
Thus, a slope of $0$ means that there is no change in y-value.
This also means that all points on the line have the same y-coordinate.
Points with the same y-coordinate belong to the same horizontal line.
Thus, a slope of $0$ means that the line is horizontal.