Laws that say what kinds of activities (industrial, commercial, residential) are allowed on your property. Zoning laws vary from town to town and state to state. In some areas, unless your land is zoned agricultural, you can't have chickens.
Sometimes you can get a variance. Or you can petition to get the local laws changed. Sometimes moving is the only answer.
I live in an unincorporated area of the county (as in I am not within city limits). It is a suburban area. I can have ducks or chickens with no problems. If I lived in the city of Fountain (to the South of me) I wouldn't be allowed to have either. If I lived in Colorado Springs (to the North of me), I could have up to 10 chickens depending on square footage, but no roosters.