Hex editors open files and display their contents to you at the byte (or even bit) level, usually in hexadecimal. If you're working with some non-compressed data, you can use hex editors to change the values of single bytes and see what sort of change that results in, which tells you what information those bytes contained. Or you might already know and be making changes intentionally, whichever. I use XVI32 because it's free and functional and people like it. All you should really need is a knowledge of what ASCII and Hexadecimal are, and the rest will more or less come down to figuring out what bytes you want to edit and to what and why.