The initialization clause
The optional initialization clause is executed once and only once before any other element of the for loop is executed.
Control then passes to the conditional clause.
As the name implies, the initialization clause is used to initialize something.
In this program, it is used to declare and to initialize the value of a counter named rCnt to zero.
(I could have declared and initialized the counter before entering the loop, but didn't elect to do so. Had I done so, I could have left this clause blank, but the semicolon would still have been required.)