FileVault encrypts the data on your Mac so unauthorized users can't access your files. If you are using OS X Yosemite, then you are likely already using FileVault. When you installed Yosemite, one of the setup screens asked if you wanted to enable this utility.
To check to see if you are using FileVault, open System Preferences, click Security & Privacy, and then click the FileVault tab. To enable FileVault, click the lock in the lower-left corner to unlock it and enter your admin name and password. Next, click the Turn On FileVault button.
You'll get a recovery key, which you'll need if you want to reset your password. Be sure to keep this recovery key in a safe spot. Alternatively, you can store it with Apple. If you choose the latter, you will be asked to create three security Q&As -- three hoops to jump through should you need to ask Apple for your recovery key in the future.
After a restart, FileVault will begin to encrypt the files on your drive. You can still use your Mac while FileVault works in the background.