A slope in real life can be compared with an inclination. It is the measure of how vertical height changes with respect to horizontal length.
If a traction load comes downhill or goes uphill it has to adjust the speeds respectively according to the slope.
A common example where slope can be experienced in real life is Roller Coaster.
While going up the hill suppose an inclination of 1ft per 10ft is there then a vehicle approaching will increase its speed in the motoring mode but while going down the hill the slope will help the vehicle getting some speed due to gravitational force along the slope and it will operate in the braking mode.
Slope is important because if an airplane while landing lands vertically or horizontally might crash and hence its the pilots are asked to land the plane at certain slope or inclination with respect to ground.