InDesign has an online help function; searching for 'overset text' yields this:
Try the following experiment to see how it works: draw a text frame, fill it with Placeholder Text, then toggle to the black arrow and make the text frame a bit smaller. Hey presto, Overset Text.
In summary, 'overset text' is simply Too Much Information. Text that has not been placed. Text for which there is no room allocated in your text frames.
You can best delete it using the Story Editor (look in the help for that), which helpfully shows you what part of a story is overset -- but, would you really want to delete this? (Perhaps you would, since you never noticed it was missing in the first place.) Another way of getting rid of it is placing your text cursor at the very end of the last visible sentence in the last text frame -- the one with a glaringly obvious Warning Signal Red Plus in its lower right corner -- and press Cmd+Shft+End keys to select everything up to the end of the story, then press Backspace.
Well, no it's not a stupid question, but ID handed you the exact terms to look up in the Help.
Don't let the "This question is Not Answered." tag put you off, there are some good answers in there.