Visiting another educational, research or related institution, opening your laptop and being instantly connected to your home network - logging in with your own institution username and password.
There is an excellent introductory animation 'What is eduroam?', created by AARNet (the Australian research and education network), on Youtube: What is Eduroam ? which is worth viewing if you are not familiar with the service.
Janet UK operate the service in the UK and futher details can be found at https://www.ja.net/products-services/janet-connect/eduroam