A lemon is a sour yellow fruit. Squeeze one to make lemonade or add flavor to hot tea. Lemon is also what people call something that doesn’t work well, like a bum car.
You can use the word lemon for the evergreen tree whose blossoms become bright yellow fruit, or for the fruit itself — or even the color of the fruit. Lemons are originally from Asia, but they also grow well in the Mediterranean, Florida, and California. They like sunny places. Informally, you can also use lemon to mean "an unsatisfactory or poorly made example of something," so you could say, "Most Volvos are great, but this car is a lemon."