Hehe, Nihil wasn't clear...he was kind of making an analogy. Opening executables in Notepad won't show you much of anything useful. The idea is the AV scanner looks at the data looking for strings it knows are suspect or malicious (signatures) or commands, functions, or instructions that are "questionable".

That was a lightweight answer. If you really want to get further into it, you'll have to get some serious experience and education.