In a perfume description, what do the top, middle, and base notes refer to?

Are they the strength of the components of the scent?In a perfume description, what do the top, middle, and base notes refer to?
Top, middle, and base notes are determined by how long the notes last. Citrus will be lovely and bright but will come off very quickly, so they are top notes. Middle notes are also called heart notes and they are the 'heart' of the perfume. Florals are typically middle notes. Base notes will provide the base of the other notes and will outlast all the other notes of the perfume. Base notes are usually deep, woody, resinous or musky.