I've looked around, and perhaps i've missed it somewhere, it isn't in the book that I can see, but where does tetris come in? i don't find any mention of tetris except in the name of the web site.
The "NAND to Tetris" suggests that you'll learn about modern computers from the ground up: from logic gates to high level software.
You can choose to build Tetris (as many have) in Jack, the high level language that you use in a later chapter of the book, and for which you will build a compiler.