HTTPS Everywhere is a browser extension produced as a collaboration between The Tor Project and the Electronic Frontier Foundation. It encrypts your communications with a number of major websites.
Many sites on the web offer some limited support for encryption over HTTPS, but make it difficult to use. For instance, they may default to unencrypted HTTP, or fill encrypted pages with links that go back to the unencrypted site.
The HTTPS Everywhere extension fixes these problems by rewriting all requests to these sites to HTTPS.
HTTPS Everywhere can protect you only when you're using sites that support HTTPS and for which HTTPS Everywhere includes rules.