The carrot is a food plant that yields a harvest in the same year it is sown. In the first year the plant germinates and develops the leaves and the root we eat. The plant stores energy in the root.
If the carrot is left in the ground over winter, something different happens the following year. Then the energy stored in the root is used to produce new leaves, flowers and form seeds, as shown in the illustration. After the seeds have matured, the plant dies.
Even though we harvest the carrot after one year, the plant is technically biennial because its life cycle, the time from seed germination until the plant produces its own seeds, lasts two years.