Where can I read about global variables?

To start, I'm sorry for asking a questing that does not belong on this website. I just don't know where else to ask it and I figured that it is at least scripting related.

I've been scripting for a few months now. In that time, I've of course come across local variables, defined as local variableName = variableValue as well as global variables defined without local before variableName = variableValue

By chance, I've recently stumbled upon a whole new side of global variables. For example _G. All I know about these types of variables is that they can be read and changed by multiple scripts but I really want to learn more. I've searched far and wide for a tutorial covering variables like _G with nothing to show. All I've found is tutorials on "regular" global variables like the one described in the second paragraph. I have found one tutorial where _G was mentioned, but he never said anything about it other than that it exists.

So, I've come to this website to ask if you know any good tutorial(s) where I can learn about these types of variables or if you can give me a walkthrough on them.