Initialize counter and accumulator and perform a priming read

The function in Listing 3 begins by setting the grade counter and the grade accumulator to zero.

Perform a priming read

Then the program instructs the user to either enter a grade, or to enter a -1 to quit.

Then it reads the input provided by the user.

(This initial input ahead of the loop that follows is commonly called a priming read.

This terminology is taken from the idea of priming a water pump to cause it to start pumping water.

If you don't know what it means to prime a pump, don't worry about it.  Understanding water pumps isn't critical to understanding this program.)