Your Master PIN is used to verify your identity when making changes to your account, and for accessing your account by phone. Grasshopper automatically generates one for you when you sign up, and you can change it by going to Settings, click Your Account, and scroll down to Master PIN (under the Credit Card Info section.)
If your interface looks different than the one above, follow these instructions for locating your Master pin:
Go to Settings > Main Greeting > Under Options, you'll see Phone Pin. This is your Master PIN.
Keep in mind! Your Phone Pin is not the same as your Master Pin, unless of course you made them both the same.