BleepingComputer.com: Explain to me

I downloaded the XVI32 hex editor some time ago, just out of pure curiosity. I read somewhere that they are good for editing programs, but all i get is gibberish. As i understand it, programs are just line after line of processor instructions, with the ocational instance of plain text. I usually get about the same when opening a dll with notepad.
First of, i want to know exactly what it is a hex editor does. Does it like, read 8 digits and interprets that as one symbol, or maybe its 16. What does it do with the actual information from the HDD?
Second, what sort of files can i open it with?

Hex Editor

okay, so a series of 8 bits is interpreted as one single symbol, represented by one character in the editor. The editor displays it as a two-digit hexadecimal value, since the hex number FF which is the highest two-digit hex number is equal to 255 decimal which is equal to 11111111 binary which is the highest eight-digit binary value.

But that doesnt explain what use i can have for such a software.

Most people would have no practical use for a hex editor.

You can use it to examine and edit the contents of absolutely any file, but that requires you to know what changes can be made to the file to achieve any purpose. That is, for an executable file you would need to understand machine code instructions, and what would happen if you changed any of them (disaster usually...). As you note, executable files often contain text, and it's possible to alter this to change maybe menu labels and the like. A few programs may use what are called string literals, and in some cases these could be judiciously altered to change some aspect of the program operation. But as far as I'm aware this would only apply to old, simple styles of program, e.g. DOS programs.

A hex editor can also read and edit absolute sectors from a disk, so could for example be used to manually edit a boot sector. But again this requires you to already know what changes can be made to achieve a desired effect such as a repair.

Thanks a bundle for the info!