In short it is the secure version of the 'HTTP' system which sends data around the web enabling (among other things) web browsers to view content from websites.
Why is it better than HTTP?
It uses an additional layer to an HTTP connection, which ensures that any data sent to or from the website is encrypted. Even if no forms or personal details are involved it is still a significant security improvement reducing attack vectors such as faked delivery of data to a browser (which can result in malware or hacks), in short - HTTPS should be used everywhere.
In addition to the security improvement Google has strongly pushed HTTPS and as a result now gives it a notable advantage in the ranking of search results meaning HTTPS pages perform better in search.
Also Google has got more and more aggressive in pushing for HTTPS. In late 2017 it began to phase in 'Not Secure' messages in search results and in the Chrome browser for any pages not on HTTPS. In the next month or two they intend to make this the standard for ALL results and users of Chrome so now is the time to get this switch completed.
What does it mean for me?
This should have zero noticeable impact for you and your use of courtauld.ac.uk beyond seeing a nice notification in your browser bar that you are securely connected (this usually takes the form of a green padlock and confirmation that the page is provided by 'Courtauld Institute of Art', as confirmed by the security certificate IT have in place for the site).
All HTTP links automatically forward on to their new HTTPS version (and our analytics and Google properties have been updated accordingly) however best practice will now be to type 'https://' rather than 'http://' whenever sharing links with anybody as that is now the correct final destination.