Honeybee foraging goldenrod.
Pollen. It's essential for flower fertilization. Bees collect it from the male stamen and transfer to the female style. They may do this in a single flower, in multiple flowers on the same plant, or between different plants (cross pollination). In the process of transferring pollen, they also collect it in sacs on their hind legs.
Protein is brought into the hive and packed in cells where it is stored. It's fed to baby bees along with nectar (carbohydrates). You can see from the pictures that pollen color varies with species.
Like you and me, bees need protein and carbohydrates. Varied sources are required for a balanced diet. Some protein sources, like almonds, are high in protein content. Other species, like pine, have very little protein for bees. Pollen also contains fats, lipids, and minerals. Bees produce enzymes to break down these components to reduce bacteria and prolong storage in the hive, typically a few months.
Nectar and pollen.
(Chart is courtesy the North Shropshire Beekeeping Association).