Cid is carrier id, and mid is model id. They are used for identification and protection purposes. Most all files you flash have an android info file which gets checked against your mid and cid. If they do not match, the file will not flash to prevent posible damage.Mostly people change them in order to flash roms or software not really intended for their phone. Some people also change cid in order to recieve an ota not yet available on their carrier, or to switch their phones to be a google play edition phone. Unless your positive what your doing, you shouldn't mess with either.