Explanation:
the slope is the inclination of a line, or of the tangent (also a line) on a point of a bent curve.
it is expressed as ratio (y coordinate change / x coordinate change) when going from one point on the line or curve to another point on the line.
it tells us how many units y changes when x changes a certain number of units (like 1).
it is positive, if the line goes up (when going from left to right). the larger the value, the steeper the line goes up.
it is negative, if the line is going down (again, going from left to right). the lower the value, the steeper the line goes down.
a horizontal line has a slope of 0/x = 0.
a vertical line has a slope of y/0, which is undefined, or sometimes expressed as infinite.