In effect, disk encryption scrambles everything stored to your computer, requiring a password or other approved recovery tool to decode it. So, if your computer falls into the wrong hands, all won't be lost.
To summarize, there are lots of encryption and other security options out there. Some are quick, easy and free. Others are going to cost money for specialized software, hardware or both.
To find a level of security you're comfortable with, start by poking around with security settings on your browser, e-mail client and favorite websites. Then consider whether you want professional help to get to the next level.