Chickens always have a 'leader' and then usually a second 'leader' who is under the rule of the first leader but is on top of the other chickens under it. Does that make sense? Let's say you have four chickens (no Roo). Chicken #1 is leader - top of the pecking order Chicken #2 is lower than chicken #1 but higher than chickens #3, 4 and 5. and so on down the line. The top chicken gets the first choice at the feeder and waterer, etc. There is always a chicken at the bottom of the pecking order. That one will often be pecked on in times of stress or being too crowded or bored. If you have a Roo he will be the leader of his flock of girls. If there is no Roo then a hen will step up and become the leader. If you have more than one Roo - they will fight it out to see how is top dog ... or Roo!
Because pecking order is so well explained in the Hobby Farms book, Chickens - Tending a Small-Scale Flock for Pleasure and Profit I'm copying it: "In any flock of chickens, there are birds who peck at other flock members and birds who submit to other flock members. This order creates a hierarchical chain in which each chicken has a place. The rank of the chicken is dependant upon whom he pecks at and whom he submits to. He ranks lower than those he submits to and higher than those he pecks at. A flock of chickens generally has their pecking order up and running by the time they're five to seven weeks old. Pullets and cockerels maintain sepereate pecking orders within the same flock, as do hens and adult roosters. Hens automatically accept higher-ranking roosters as superiors, but dominant hens give low-ranking cocks and uppity young cockerels a very hard time. In a closed flock with an established pecking order, there is very little infighting. Each chicken knows his/her place, and except among some roosters, there is suprisingly little jostling for position. Dominant chickens signal their superiority by raising their heads and tails and glaring at subordinates, who submit by crouching, tilting their heads to one side, and gazing away---or beat a hasty retreat. The addition of a single newcomer or removal of a high-ranking cock or hen upsets the hierachy and a geat deal of mayhem erupts until a new pecking order evolves. Since brawls are invariably stressful, it's unwise to move birds from coop to coop. Because low-ranking chickens are shushed away from feed and water by bossier birds, they rarely grow or lay as well as the rest. Indeed, low-ranking individuals sometimes starve. If pecked by betters until they bleed, they may be cannibalized by the rest of the flock. It's important to provide enough floor space, feeders, and waterers so underlings can avoid the kingpins and survive.