Frozen in place because something has completely caught you. You are staring, you are not moving, and for that moment you could not look away even if you wanted to. The feeling that holds you can be good or bad — deep interest, wonder, shock, horror, fear. A child can be transfixed by a magic trick; a driver can be transfixed by a crash at the roadside. The key idea is not just 'very interested' but 'held still' — the body stops. It is usually used as an adjective after 'be' or 'stand' or 'sit': she stood transfixed, they were transfixed. It is often followed by 'by' for what caught you. The word is a little dramatic and belongs to writing and careful speech more than to casual chat. Its force comes from where it started: to transfix once meant to pin something in place by pushing a spear or sharp point right through it. That old picture is still there under the modern meaning — you are pinned to the spot.