The IMSI (International Mobile Subscriber Identity) is a unique number associated with a cellular network subscriber, stored securely on the SIM card, used to authenticate the user's billing account and phone number.
Users often confuse the IMSI with the IMEI. While the IMEI identifies the physical phone hardware, the IMSI identifies the subscriber. If a device is stolen, the carrier blacklists the IMEI to render the phone useless, but they will issue a new SIM card containing the user's original IMSI so they can seamlessly retain their phone number.
