Pollen is the high protein food which the bees use to make food for their brood. If you see pollen coming into the hive then it's reasonable to assume that there is brood being reared in the hive.
The bees visit flowers and collect the pollen in 'baskets' of hairs behind their hind legs. We can see these 'baskets' as bright patches on the legs of returning bees. The lumps of pollen sometimes get dropped and fall to the floor of the hive where they can be examined.
Pollen is an indicator of which flowers are in bloom at any time. It isn't a guarantee that the bees are getting nectar from those flowers. Nectar flow can vary by plant and conditions. Wind pollinated plants produce no nectar at all (as far as I know).
Pollen can also be eaten by humans if you like that sort of thing. I know one small person who likes it. It has a pleasant, subtle flavour -- sometimes with an aroma of the flower. You won't get well fed on it though. The amount of pollen in each basket is tiny.