The molecules of each ingredient in a fragrance evaporate from the skin at different speeds, according to their size and volatility - all the while interacting both with the fragrance's other ingredients and with the wearer's skin chemistry. In perfumes like Ffern's that use natural ingredients, ingredients may contain many hundreds of different molecules, all of which contribute to the overall scent profile of a fragrance.
Mid notes are also called heart notes, and it is indeed these notes that nestle at the heart of a fragrance and determine its overall character. Emerging some hours after the fragrance has settled, their molecules are heavier and less reactive than those of top notes. Mid notes are often richly floral or peppery, including rose, jasmine, and black pepper.