i++ increments the value of i by one and then stores it back in i. It is a quick way of doing:
i = i + 1;
If you look through the Pololu AVR library code you should be able to find a line that sets _numSensors. If I remember right, it is a global variable that probably gets set by the QTR init function.
For a basic introduction to the C language, take a look at this C tutorial (you can skip sections on C++).
You also should take a look at nexisnet’s post in this forum thread:
