Unlike perennials that flower year after year and annuals that have a one-year life cycle, biennials have a two-year life cycle. In the first year the plants grow roots, stems, and leaves. Then, in the second year the plants come into flower, before producing seeds and dying.
Due to them not flowering in the first year, it’s great to plant them among perennials. Alternatively, plant other biennials in the year that the first biennials are coming into flower to ensure continuous flowering.