A biennial plant is similar to an annual plant in that they are typically planted as a seeds or a started plant each year in the spring and harvested that same growing season. What makes biennials different from annuals is that they complete their life cycle during two growing seasons.
In the first year, they do not produce flowers or set seed. This occurs in the second year. Some plants can overwinter directly in the garden, usually under a thick mulch, while other plants like carrots, beets, and other root veggies are pulled up and out of the ground, stored over the winter, then replanted the following spring and allowed to flower and set seed.
Examples of biennial vegetables include beets, parsley, parsnip, leeks, and celery.