To be spellbound is to have your attention captured by something, captured so strongly it feels like witchcraft.
A spell is a type of magic that controls your mind, and being bound is being tied to something. Being spellbound is when your attention is caught by something, and you just can't look away, almost as if you were bound by a spell. An exciting movie can leave you spellbound. A great public speaker can hold the audience spellbound. When you're spellbound, your attention is on something 100%. Being spellbound is like being fascinated, hypnotized, mesmerized, or transfixed.