What does "collect words" mean?

I wanted to turn off auto-capitalization, so I went to Tools>>AutoCorrect>>AutoCorrect Options, and I disabled it.

However, as I was poking around in there, I saw the Word Completion tab and a checkbox for enable/disable Word Completion. Great, I don’t want that, either. So I disabled it.

Underneath that was “Collect words” with a checkbox and the default is apparently enabled.

What does that mean, “Collect words?” I searched it and couldn’t find any explanation. Under that there was another checkbox for "When closing a document, remove the words collected from it from the list. That’s disabled by default.

What the heck IS this stuff?

Thanks

Why are you asking us about “this stuff”, why did you press so many buttons?

Thanx.

You’re a real prince.