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.