Username / Password
In rare cases a mobile carrier may require a username and password in order to access data. If this is the case, enter that username and password in these fields.
In most cases this will be left blank.
This username and password is used to authenticate the PPP (Point-to-Point Protocol) connection from your phone to your carrier's servers. It is rare that a carrier would require this, as they can already identify you in a more secure way from the information in your SIM card, and using a username and password does not provide significant additional security. In cases where a carrier does require these fields, they will usually just require a rather general word in both, rather than anything unique.
In general, you shouldn't fill these with the username and password you use to access your account on the carrier's website. It is usually unrelated.
Note: Despite what their positioning might suggest, these fields do not relate to the Proxy fields above them, and do not represent the username and password given to the proxy server. Unfortunately, Android does not support HTTP proxy servers that require authentication.