This function, along with its siblings,
_satellite.removeCookie, were provided in DTM to perform basic client-side cookie operations.
This, and the others mentioned above still exist in Launch, but have been deprecated and will be removed at some point int the future. The best guidance it to replace them with the new set of cookie utility calls in Launch.
//Crusty old DTM way
_satellite.readCookie("favoriteThing"); //might return "kittens"
//Shiny new Launch way
_satellite.cookie.get("favoriteThing"); //might return "kittens"