Now we will let our Arduino do the hard work for us. The Arduino really is a computer, so we'll let it do some repetitive calculations.
This line of code does just what it appears to say. This line of code can appear confusing if you think about it in the math sense, but this is not an algebraic expression. In plane english this says, "Set the new value of ledPin to be the old value of ledPin minus one." The Arduino is only capable of simple arithmetic. If you want it to do algebra you'll have to program it to do it.
So with "ledPin=ledPin-1" in our program the value of ledPin will decrease by one each time through the void loop(). After a few times through it will be lower than our lowest pin and in fact will eventually be a negative number unless we do something about it!
The statement above asks a question. It asks, "Is the value of ledPin less than nine?" there are two possible answers to this question, yes or no. The Arduino thinks of these as True or False. If the statement is false the program will skip whatever is in the curly brackets and continue on. However, if the statement is true it will execute the code in the brackets and then continue on. So, in our case if the value of ledPin gets too small we will re-set the value of ledPin to 13.
After you complete the program above.