Many websites still do not use full HTTPS (the “S” stands for “secure”) or have only partially secure sites that feature un-encrypted links. That means that any communication between you and those sites can be captured by third parties. That would suck.
In their long-term project to create a fully encrypted web, the EFF has created HTTPS Everywhere, a browser extension that rewrites how requests are made and defaults to HTTPS when possible. This is a great idea.
Sadly, the internet is still full of un-encrypted and non-secure content. HTTPS Everywhere is not going to fix all your vulnerabilities on the web, but it will eliminate a real problem with an easy, easy solution. This is the easiest solution to any problem ever.