To heat up means to become warmer or to make something warmer. When you heat up leftovers in the microwave, you're raising their temperature so they're nice and hot to eat. On a sunny afternoon, a car's interior heats up quickly when parked outside.

The phrase also describes situations that become more intense or exciting. A close basketball game heats up in the final minutes when both teams play harder and the crowd gets louder. An argument heats up when people start talking more forcefully and emotions run higher. A competition heats up when rivals start performing at their best.

In sports, when a player heats up, they suddenly start performing extremely well. A basketball player who heats up might make five shots in a row. A slugger who heats up at the plate starts hitting home runs after struggling for weeks.

Sometimes people use “heat up” to describe getting more active or energetic before the main event. A band might heat up backstage before a concert, or an athlete might heat up with practice swings before batting.