Heartburn is referred as a condition when the digestive acid flows into the esophagus from the stomach. The esophagus which is situated in the heart area of the chest feels irritated accompanied by a burning sensation.

Spicy foods for one.

Is this homework?

Heartburn is actually more often used for the gas that builds up in the highest part of the stomach, which is right under the heart. Acid reflux is what you are talking about, or GERD, although all 3 terms are used loosely for any acid stomach upset.
Foods to avoid are spicy, caffeine, chocolate, overeating in general, but really anything that bothers someone. Speaking as someone with heartburn and acid reflux.