Forms Mode lets you view and make changes to your tax forms "behind the scenes."
If you're adventurous, you can even prepare your return in Forms Mode, but we don't recommend it. You may miss obscure credits and deductions you qualify for, and you may forget to report things that will come back and haunt you later.
Forms Mode is exclusively available in the TurboTax CD/Download software. It isn't available in TurboTax Online.