This code operates by having the preprocessor evaluate the expesion 0, which is always going to evaluate to be false, and the code in the if statement will be ignored.  This is a better way to comment out huge blocks of code than /* */ because it nests and is more obvious that it's a temporary removal of a chunk of code.

Code:

#if 0

Useless code;

More useless code;

Even more useless code;

#endif 

Extensions:

I would make sure to leave a comment with the #if 0 explaining why it was commented out and what it was replaced it or if it's meant to be fixed and uncommented.

Sources:

None.