A generic term used to refer to a chicken cut into parts. Even though technically it includes the breasts, because they are currently the most prized and expensive pieces of a chicken, in practice the breasts aren’t included in the current meaning of the word: when a recipe calls for chicken pieces, it will generally mean cheaper parts of the chicken. Come to think of it, in this sense, it used to include chicken wings, but now that they are worth their weight in platinum, they too are probably excluded in general practice.
Pieces would be the remaining parts: the legs (consisting of the thigh and drumstick), the neck, the back, or the innards. No doubt these too will someday become fashionable, and Chicken Pieces will be reduced to meaning bags of plucked feathers.
You’ll have to figure out what Chicken Pieces are best to use based on the recipe you are looking at: if it looks like you are expected to end up with some meat, you’ll probably want the chicken legs. If it looks like the Chicken Pieces are just being boiled to make a flavourful water, then it’s too wasteful to use the legs: you’ll want necks or backs instead. The backs contain a lot of flavourful marrow for stock.