No one is sure why the letter "m" is used to represent slope in the equation of a line. While there are many theories, the variable may simply have been arbitrarily chosen and used as a standard convention.
The slope of a line describes what angle it lies at. The slope can be calculated by dividing the quantity of change in the "y" direction, otherwise known as the rise, by the quantity of change in the "x" direction, otherwise known as run, over any segment of the line. Therefore, a high slope indicates that the line extends upwards quickly, while a low slope indicates a flatter appearance.