If you're planning on securing data on your local device, you could choose to just encrypt a few key files, or you could encrypt everything on the computer. “Full disk encryption” is the term for encrypting everything. It's usually safer (and often easier) to use full disk encryption than to manage just a few individually encrypted files. If you try to encrypt just individual files, your computer might make temporary unencrypted copies of those files without you noticing. And some software might keep some unencrypted records about your use of your computer. Apple's OS X, Linux and high-end versions of Windows all have built-in full disk encryption, but it is usually not turned on by default.