Allergens are ingredients commonly found in cosmetic products, clothing, and lots of other materials used in consumer products (on mobile phone casings, laptop sleeves, gym equipment handles, slippers, clothing, underwear, goggles, etc.) and in life in general (plants, pets, etc.) that are known to cause allergic or irritant reactions in skin.
Common allergens are those that tend to cause reactions the most. These are determined by studies that examine the patch test results of thousands of people in multiple clinics and testing centers in different countries.
A common misconception is that "natural" and "organic" ingredients are not allergenic. In many cases, it is the opposite that's true. Bee stings, mangoes, shellfish, pollen, dander, peanuts are examples of different things that, even in their most natural, organic forms, can cause allergic reactions (some very severe) in people who are sensitive to them. And many natural ingredients are on the allergen list.