A headliner is the main act. At a rock concert, you usually have to sit through an opening band or two before the headliner comes on stage.
The headliner is the biggest star or the main event. The headliner at a jazz festival is the most popular performer, the one most audience members came to see, and the headliner of a musical theater review is the star of the show. In the early 1890's, a headliner worked for a newspaper — the word originally meant "one who writes headlines." By the end of that decade, it had come to also mean "one who stars in a performance."