HTTPS provides a combination of the HTTP and SSL protocols, enabling encrypted communication between your computer and a web server. Without it you’re exposed to sniffing attacks on the network; for example, if you’re using a public Wi-Fi to access Facebook via plain HTTP, someone using the Firesheep add-on for Firefox can easily retrieve your data. HTTPS makes it a lot harder to do that.
The feature is available as an option on the Account Settings page. If you don’t see it yet, don’t worry; Facebook will be gradually rolling it out over the next couple of weeks.
Unfortunately, even Facebook’s implementation of the protocol will not be perfect. “Some Facebook features, including many third-party applications, are not currently supported in HTTPS,” writes Facebook’s Alex Rice in a blog post, promising that Facebook is working hard to resolve these issues. Furthermore, if you choose to enable HTTPS, you should know that it will slow down your Facebook experience, as encrypted pages take longer to load.