The SLOPE function returns the slope of a regression line based on known y values and known x values. A regression line is a "best fit" line based on known data points.
The slope of a line is a measure of steepness. Mathematically, slope is calculated as "rise over run", or change in y over the change in x. For example, if a line has a slope of 2/1 (2), then if y increases by 2 units, x increases by 1 unit.