what does x reprsent in this code ?

[ndc85430]

What book are you reading? Think Python is pretty good: http://greenteapress.com/thinkpython2/html/index.html. Do note that when you learn programming, you're really learning a set of tools and it's up to you as the programmer to work out how to use them to solve whatever problem you have (e.g. the problem of "writing a Sudoku game").