An etrog is one of the Four Species, which are four different kinds of plants that have symbolic significance during the Jewish holiday of Sukkot. The etrog is a kind of citron and is considered the most important of the four plant symbols because it has both fragrance and taste. Of the other symbols, a palm tree bears fruit but has no fragrance, a myrtle tree has fragrance but bears no fruit, and the willow tree has no fragrance and bears no fruit.
Because the citron is the only plant symbol that is both fragrant and edible, it has special significance when it is waved together with the other symbols during Sukkot. The other plants are bundled together to form a "lulav," which is held in the right hand, while the etrog is held in the left hand. Why the left hand? Because it is closest to the heart.