The slope of a line is a measure of the "steepness" of the line. On a Cartesian plane, the slope of a line can be computed from any two points on the line.
Slope = ( y1 - y2 ) / ( x1 - x2 )
where x1 and y1 are the x and y coordinates for the first point; and where x2 and y2 are the x and y coordinates for the second point.
A positive slope is indicated by a line that goes up as you move from left to right along the X axis. A negative slope is indicated by a line that goes down as you move from left to right. And when the line is horizontal, the slope is zero. A line with zero slope is parallel to the X axis.
In the figure above, the green line has a positive slope; the blue line, a negative slope; and the red line, zero slope.
In the equation for a straight line, shown below, slope is one of the key parameters:
y = mx + b
where m is the slope of the line, b is the y intercept of the line, x is the independent variable, and y is the dependent variable.