In Wireless communication, it is unavoidable for the signal from transmitter to go through noisy and fading media(environment) and get corrupted and distorted. So there should be a special mechanism to remove(at least reduce) the noise and compensate the fading at the receiver side. One common way to do this is to use a specifically designed signal which are known to both transmitter and reciever. The transmitter transmit the known signal and the known signal also goes through noisy and faded media and eventually reach the reciever. The reciever detects the special signal and check how the signal get distorted and figure out how to 'undo' (compensate) the distortion. This specific signal is called reference signal.
Different Radio Technology use different types of Rerence signals. Since there is no single type of reference signal that fits every types of signal distortion (e.g, phase noise, timing error, amplitude noise etc), the experts designed various types of reference signals depending on specific purpose.