Verbascum thapsus is an herbaceous biennial that forms a rosette of large hairy leaves in the first year, with a spear of yellow flowers in the second year, each with hairy stamens that provide landing spots for visitors such as the bees and hoverflies. These hairs can be seen in pink on Verbascum nigrum. 

Each flower opens before dawn and closes in the same afternoon and will not open again, a succession of flowering takes place up the spear. The flowers are protogynous meaning the female parts mature first. This process allows time for cross pollination before self-pollinating if unsuccessful. While many insects visit the flowers, including the mullein moth caterpillars (Shargacucullia verbasci) which can be seen in abundance eating, only certain bees complete pollination.

In order to flower, the plant requires winter dormancy. The low temperatures allow the breakdown of starch in the roots. However, if low enough temperatures are not met, Gibberellins (plant hormones) can complete this task. After flowering, the entire plant usually dies at the end of its second year.

This plant has many medicinal purposes mainly known for its management of respiratory disorders such as: asthma coughs and tuberculosis. Also having both demulcent and emollient properties soothing the skin, ideal for rash treatment or inflammation.
as can be seen here:
Disclaimer: Do not self administer herbal medicine without doing your own research and consulting a GP prior. I do not advocate traditional medicine.