Under Overture 3, you have to first click the Playback tab under Options/Preferences, though they might have moved this somewhere else with Overture 4. JHMO, automated ornament playback almost always sounds awful, though I suppose it could be used to give one at least some idea of what the effect might sound like.
I've not used this option much, but I would suspect it means exactly what it says. It will play trills and tremolos, but probably not mordents and turns. I'm not even sure how it knows how to play whole vs. half step trills, unless it just goes by the key signature, which is often not the case with a given trill.