If you're entranced, you are charmed and mesmerized by something. An entranced theater-goer might gasp out loud when something scary happens on stage.
When you're entranced by something, it's got you under its spell. In fact, the earliest definition of the word was "put into a trance" or "put under a spell," with the roots en-, "put in," and trance, "state of suspense" or "state of insensibility to mundane things," from the Old French word transe, "fear of coming evil." The meaning of entranced has shifted over the years to be more positive — more "charmed" than "cursed."