Bi-ennials are plants which take two years before producing seeds, flowering or growing fruit.
In their first year they will develop roots, generally a thick, deep tap root; plus stems and leaves, often referred to as a ‘basal clump’, or low mound of foliage.
If the plant is a weed, or just generally not in the right place in your garden, this can be a good time to dig it out, complete with its long tap root.
Honesty (Lunaria) a cottage garden favourite, is an ornamental bi-ennial.
However, some of the food crops we grow are also bi-ennials, such as carrots. We eat the roots of the plant as they are full of sweet, sugary starch, that the plant would otherwise use as energy in its second year.