Do you confuse the terms "biennial" and "biannual"? That is understandable. The latter adjective is defined as pertaining to something that happens every two years; when dictionaries define it, they often list the former as a synonym.
When the subject is plants, however, "biennial" is the pertinent word. This adjective is defined as pertaining to a plant that lasts for two years. That is, such plants complete their life cycle in a two-year period. What mission must a plant accomplish in its life cycle? It must reproduce. Biennial plants bloom and set seed in their second year, then die.
Used in this sense, "biennial" also functions as a noun. We may say, for instance, "The following plants are considered biennials," and then proceed to list examples:
But note that the descriptions for some plants will say, "may be a biennial or short-lived perennial." My first entry above (foxglove) is an example of a plant whose status isn't entirely clear-cut; so do not be surprised to see such a plant listed as a perennial somewhere.
To teach beginners what "biennial" means, it is sometimes easiest to answer two related questions first:
Biennials can be seen as plants whose status places them in-between annuals and perennials.