If you don't know what the "S" in HTTPS means, it's simple "S" is Secure. Because when you connect to a website that does not use HTTPS, everything sent to and from (for example content of forms) you is sent in plain text. And so almost anybody (including your internet service provider or government representatives) can, for example, see all sites you visit. But this is not possible on HTTPS. Use it, it's much more secure. And to improve your security a little bit more, try browsing only sites that use HTTPS.