Pollen is essentially the "male" reproductive cell of a plant. In order to produce seeds, it must travel to the "female" part of the plant, the stigma, where it can travel down the pistil to the ovule, and produce a seed. The seed then has the correct number of chromosomes to produce a new plant.
Some plants self-pollinate; the pollen fertilizes the female part of the same plant that produced it. But for others, the pollen must travel to a new plant, which may be quite far away. There are several ways this can happen. The wind can carry pollen to new plants. People can intentionally pollinate plants in order to cross the particular plants they want. Many plants rely on some type of animal pollinator. These plants tend to be brightly colored or have a strong smell, in order to attract the pollinator. Bees, butterflies, moths, flies, hummingbirds, and even bats can act as agents of pollination. They are attracted to a plant because of its color, smell, or both. While visiting the plant, they pick up the sticky pollen from one, which they then carry to the next plant they visit. The seeds can then be formed as the pollen is deposited on the stigma of another plant.