Here the topic is "functions." Roughly speaking, we have a box. Input enters at one end and output exists at the other. Now, if every input value x (such as 2) is doubled by the box, every output value is 2x: (0,0), (1,2), (2,4), etc.
We call this "doubling" a "constant rate of change.
If you plot the points given, you'll see that they all are on a straight line. That straight line has the slope 2: y = 2x. Again, 2 is the "constant rate of change" here.
If you drive at a steady 55 mph, you are covering 55 miles during every hour behind the wheel. Again, 55 is a "constant rate of change."
Definition 2 (Constant rates of change) The (constant) rate of change with respect to the variable x of a linear function y = f(x) is the slope of its graph.
