From my understanding, it's a ewe who produces good wether lambs, but probably wouldn't produce good breeding ram lambs. I think the term is mostly used by people in the show sheep industry. Maybe some others will chime in.
Also, I know you're not brand new but still wanted to welcome you to BYH !
Thank you for the welcome back, it's been awhile! I still have a few questions about wether type ewes, but will save them for when I have a little more time to word them appropriately.