Pollen itself is the chalky substance in flowers. Every season, except for winter, it's released to fertilize other plants of the same species. Insects, birds, animals, or wind can spread the pollen. In general, the spread of pollen is a good thing for plants because it helps them grow. But for people with allergies, it means nothing good. And for asthmatics, it can aggravate their symptoms.
How does the allergy from plants feel? If you are allergic, you will probably experience sneezing, runny nose, tearing, etc. These symptoms happen every year around the same time when pollen is in the air (around 2-3 weeks). Also, it's easy to trace what exactly causes your allergy. If spring is challenging for you, you are allergic to tree pollen. Allergy to grass is more common in summer. And weed pollen is more of a fall issue.