Macro Extension Tube Sizes – Photo Stack Exchange

I am looking for extension tubes for macro photography and I do not quite understand how the length of the extension tube works, combined with the actual lens (for example, the standard 18mm-55mm ).

I may not be able to explain it perfectly (because I'm confused about it), but hold on to me …

So basically, if you have the normal lens set to 18mm and add a 21mm extension tube. What effect does it have?

I realize that there is a point where the camera will not be able to focus if you match an x ​​length extension tube with the standard lens of length x. Can any one summarize that?

I apologize if this sounds like a simple question and I've already read quite a lot about it, but I'm still puzzled about this part.

Any help appreciated.