When y value of a function f(x) remains constant for certain amount of increase in x value and then jumps to a higher value at another x value, then the function is called a step function This means that y value remains constant while x value increases and then at a certain point it suddenly takes a higher value.
Postage rates change the same way. Usually postage rate remains constant for a period of two to three years and then jumps to a higher rate. For example, around 2012, first class mail stamp was at 43c . Then around 2014, it increased to 49c. This a step change of the rate from 43c to 49c between 2012 through 2014. So the y variable is the postage rate and x variable is the year.