In skiing or walking you can tell where the path is going up, down or is on the level. The slope is positive on uphill portions, negative on downhill portions and zero on flat portions. Knowing the sign of the slope gives information about the hill. The slope changes from positive to negative in crossing a hilltop. Just knowing the sign of the slope is enough to identify the uphill, downhill and flat portions of the path, and then location of high points and low points.
Positive slope corresponds to going up while negative corresponds to going down.
Moreover, and this is whether matters become complicated, the slope in changing may increase or decrease. Here a positive slope may increase by becoming more positive and a negative slope may increase by becoming less negative. Likewise, a positive slope may decrease by becoming less positive and a negative slope may decrease by becoming more negative. And in all these cases, the steepness or slope of the curve changes. The steepness is given by the absolute value or magnitude of the slope.