Slope is a measure of the steepness or gradient of a line and is calculated by finding the rise over the run between any two points on the line. This video explains how to calculate the slope of a line using two points on its graph by finding the change in y-values (rise) over the change in x-values (run). It also shows how to use the slope and a point on the line to write the equation of a line in y=mx+b form, where m is the slope and b is the y-intercept.