From Sega Retro
A hex editor (or binary file editor or byte editor) is a type of computer program that allows a user to manipulate binary (normally non-plain text) computer files. Hex editors that were designed to edit floppy or hard disks directly are sometimes called disk editors or sector editors.
Using a hex editor a user can see or edit the raw binary data that a file contains, as opposed to the interpretation of the same data that other, higher level application software presents. For example, this could be raw image data, in contrast to the way image editing software would interpret the same file.
In most hex editor applications the data of the computer file is represented as hexadecimal values grouped in two groups of 8 bytes and one group of 16 ASCII characters, nonprintable characters normally are represented by a dot "." in the ASCII part.