Final answer:
A horizontal line has a slope of zero since it's flat, and a vertical line has an undefined or infinite slope due to vertical movement without horizontal change. The slope indicates the steepness and direction of a line on a graph.
Step-by-step explanation:
If a line is horizontal, then its slope is zero. This is because a horizontal line has no vertical change no matter how much we move along the x-axis; it is completely flat. For a vertical line, the slope is considered undefined or infinite because a vertical line rises or falls infinitely for any horizontal movement; mathematically, this corresponds to a division by zero, which is undefined.
When discussing the slope of a line, if the slope were to increase, it means that the line would appear steeper on the graph. Conversely, if the slope were to decrease, the line would appear flatter. This applies regardless of whether the slope is positive or negative; a higher positive slope indicates a steeper incline, while a larger absolute value of a negative slope indicates a steeper decline. The slope is a measure of steepness and direction of a line on a graph.