The document defines and provides examples of different types of slopes:
1) Positive slopes incline from left to right, like a rooftop or boat sail. Negative slopes decline from left to right, like a valley or airplane tailfins.
2) Steeper slopes have a greater incline and are almost vertical, like a triangle. Flatter slopes have a smaller incline and are closer to horizontal, like a bridge ramp.
3) A zero slope is a horizontal line with the same y value for each x, like a horizon line. An undefined slope is a vertical line with the same x value for each y, like a light pole or tree trunk.