The Pecking Order is a term used to describe the social hierarchy of chickens. The main use of the Pecking Order is to describe the dominance of one chicken, or others, to the rest of the flock.
One of the first things chickens will do upon being greeted to one another, is to determine who is the dominant hen or rooster. This is usually done by fighting, mainly pecking. The chicken that dominates the rest of the chickens will be at the top of the social hierarchy, and will be one of the more important ones, while the weakest and smallest chickens will be directly at the bottom, and are prone to bullying.
One the Pecking Order is established, there will be different interactions between the chickens. For example, the top chicken will always get first dibs on the food, dust-bathing spots, nests, and other locations, often pecking other chickens who try to take it first. The other chickens will be forced to endure this, and will sometimes fight with the top chicken, to acclaim their dominance, and attempt to be the new leader.
However, chickens at the bottom are usually excluded, bullied, and left last for generally many things, including food, nests, and other spots.