The difference between http and https is that with https any server page requests from your computer are automatically encrypted then decrypted by your browser using Hypertext Transfer over Secure Sockets Layer protocol.
This means that any data sent to the server is encrypted so log in data, for instance, is much more secure from unauthorized access.
There are plenty of http sites on the web still but more and more of them are switching to the https protocol as time goes by.
If you are on a http site just be careful what personal information you divulge because hackers can more easily infiltrate and steal personal information from non-encrypted sites.
That's why I was so glad to see this site "finally" start using the https protocol. Something I was advocating for in the past.