Quick Text, or QTEXT, is one of those holdovers from the days when regenerating your drawing meant a coffee break, but it's still around and can be useful.
What it does is change the text in your drawing from actual characters to square boxes. In the "old days" this saved a lot of time in rendering the text, so you could work faster. With today's processor speeds, though, the difference isn't really noticeable.
Hey, where'd that extra box come from? Turns out it's an empty piece of single-line text, one that contains only spaces. With QTEXT on, you can see these items, select them, and delete them.
This trick won't find empty mtext objects, though, and it's been made more or less obsolete by the "purge empty text objects" option in in the Purge dialog, but you never know when it might come in handy.