A hex editor is a computer program that allows the user to view the contents of a file or data stream using hexadecimal notation, which is typically the preferred way of viewing digital files. Hex editors typically support additional features for editing, searching, and viewing data in various ways.
Though I had been playing with computers since around 1990, I didn't start using hex editors until the late 1990s, and then, only sparingly. It wasn't until the 2000s that I finally started to understand the power of a good hex editor, and then started looking for more and better programs. My current favorite is HxD because it is both free and full-featured.