json files with a list of all cities worldwide(id, city, country, geo coordinates). Good for city search functionality, e.g.: weather for city functionality.
City ids are from geonames database ( http://www.geonames.org ). More about geonames db: https://en.wikipedia.org/wiki/GeoNames.
City ids also match the ids from openweathermap service (see https://openweathermap.org/find?q= ).