We've learned what a batch file is and how to write them. Now it's time to get all that batch files can bring us. It's easy to find out what it is for. The NOT word is optional, and it makes the condition inverese.
Lets see one by one.
What it does is to find out if there is or not a file. Maybe you don't want to verify a file, but a drive or a directory. To avoid lose control of the program, is useful to put every string into quotes. This evaluates the last errorlevel number present. Errorlevels are generated by programs to inform about the way they finished their execution.