The pollen count is a measurement of the amount of pollen in the air. The higher the count the more severe symptoms of hay fever can become (depending on the specific type of pollen you are allergic to).
Source: Hay fever @ NHS Choices
- Pollen count @ Wikipedia
Excerpt: Pollen count is the measurement of the number of grains of pollen in a cubic meter of air. The higher the number, the more people will suffer if they are allergic to a particular pollen (hay fever). Usually, the counts are announced for specific plants such as grass, ash, or olive. These are tailored to what is usually a common plant in the area being measured.