What Does "Signature Checkbox Checked By Default?: Mean?

I would assume it means "By default - the setting that's there if you never change anything - the box is checked. If you don't want it, then you'll have to un-check it manually."

Your "Signature" is a small line or sentence that will automatically appear at the end of every one of your posts. For example, my "signature" is currently that little line at the bottom of all of my posts from C.S. Lewis. I don't believe signatures are currently activated for newer members, so the question for newer members such as yourself is somewhat moot until then.