A perch simply a rod or ledge where your chickens will park themselves when inside the coop and not nesting in their nesting boxes.
Perches are a very simple item to add to your coop, but before just adding it randomly, I have found there are some things to keep in mind.
Proper placement of the perch within the coop and following some general construction guidelines that follow Chickens will instinctively seek a high elevation in the coop to provided added safety from predators.
So when deciding where to attach the perch, it's best to place it as high as practically possible.
Typically the perch should be higher that the nesting boxes, or your chickens will tend to ignore it.
It's also best to provide a easy way for your chickens to access the perch; sometimes a ladder plank inside will work well for this.
A good strategy might be to ladder plank up to a landing area in front of your nesting boxes(which might for example be 18-24 inches off the floor of the coop), and then from this landing area or deck they can hop up to the perch.
To estimate the length of the perch, a good rule of thumb is to provide 8 inches of length per chicken.
A 2x2 with rounded corners is a good size.
You might also consider using a tree branch of a similar diameter for a more native environment.
If you live in an extremely cold climate, it is often recommended to use a 2x4 resting flat.
This way the chicken can actually sit on there feet to keep them warm.
Since your flock will spend considerable time roosting on the perch, the area immediately below will receive a high concentration of droppings.
Some will incorporate a removable tray immediately beneath the perch to facilitate ease of cleaning.
Needless to say the food and water supply in the coop should be placed away from this area.