Lemons are a fruit, a citrus one to be exact. They are used for cooking, flavoring, and eating on their own (you can put salt or sugar on them; I myself prefer salt). The zest (the grated rind of the lemon) is a potent ingredient used by many chefs around the world. The juice from a lemon can be mixed with sugar and water to make lemonade (my mouth is watering...). The juice can also be squeezed onto fish, flavoring them nicely. Lemon wedges can be put into water or tea, and can be beneficial for one's throat (and taste buds).
Lemons are very healthy (unless you're allergic to citrus fruits...). They have a huge amount of vitamin C, and in the olden times, were consumed to prevent scurvy, a nasty and, if not treated, fatal disease.