The constant rate of change is a predictable rate at which a given variable alters over a certain period of time. For example, if a car gains 5 miles per hour every 10 seconds, then "5 miles per hour per 10 seconds" would be the constant rate of change.
When graphing the constant rate of change, it is important that the variance of the x and y coordinates move in conjunction with each other. For each alteration in the horizontal x axis, there must be a complementary change in the vertical y axis. So long as the change is consistent between the two, the rate of change is constant. If the change in the x and y relationship changes, even if it occurs as a statistical outlier, the graphing equation will no longer be constant.
If a rate of change is constant, the line goes either up or down in a straight line that follows a sloped path when it is shown on a graph. If the rate is truly constant, it will not fluctuate at any time, since that would negate the "constant" aspect of the rate, making it a variable rate of change or inconstant rate of change.
In economics, statistics and mathematics, a constant rate of change is very desirable since it means that future outcomes are very predictable.