RJ TextEd

I've never seen this behavior before. It is (apparently?) pretty specific.
It doesn't seem right to me.

I was editing an .HTML file, and I had a line with a URL on it, I was at the end of the line,
and I pressed the TAB key to tab over (I was making a columnar table of URLs),
and it changed the line:

from: bdt.com
to: <bdt class="com"></bdt>

That is an error or bug to me.

Whatever it is, I don't want it to do that and I don't think it should.

If I press SPACE first, then it doesn't do it, it just SPACE-TABS over.

This is Emmet in action, and you have tab defined as a trigger key (it is default setting, I guess). Go to Format > Abbreviations > Allow Trigger Keys and uncheck tab.

Thanks.

It was enabled.

I don't know what that stuff is, though.

Quick way of expanding snippets into HTML code, cf. https://docs.emmet.io/ (see the demo there for a quick introduction).