A heartthrob is someone, usually a hunky guy, who is so attractive that he makes our hearts throb in our chests. A heartthrob is often someone famous like a movie star.
The word heartthrob has been used to describe someone who makes people feel romantic since the 1920’s. It’s officially gender neutral, but it usually describes a man. Heartthrobs are movie stars, musicians or any celebrity that people want to smooch. It could also be the cute high school football player that all the girls have crushes on. The first cited heartthrob (1839 in the OED) was simply "a pulsation of the heart." It wasn't long till this morphed into something that causes the heart to throb (perhaps abnormally so), and soon thereafter, into someone who causes the heart to throb with desire and longing: that's what we usually mean by heartthrob today.