When using eduroam everybody needs to use an @organisation suffix on their username – the bit after the @ is known as the realm. If you are from the University of Bristol the realm can either be bris.ac.uk or bristol.ac.uk. So if your Bristol username was ab1234, you could use either email@example.com or firstname.lastname@example.org as your eduroam username.
The realm indicates to which organisation your credentials (username & password) should be sent to for verification. Without the realm, the eduroam system at each of the many organisations worldwide wouldn’t be able to verify your username and password and so wouldn’t be able to allow access to the Internet.