Pollen is the grainy stuff inside a flowering plant that makes it possible for the plant to reproduce. Insects, birds, people, and the wind help to spread pollen between plants.
When pollen spreads to the female part of a plant, it germinates, or begins the process of growing a new plant. When pollen spreads like this, it's called pollination, and it's how plants reproduce. Pollen is great for plants but not so great if you have hay fever, an allergy to pollen. The first meaning of pollen was "fine flour," which is what pollen looks like.