The full form of IMEI is International Mobile Equipment Identity. Every device has a unique IMEI number and one can easily find it out by simply dialing *#06#, through Phone Settings or from the retail packaging label.
IMEI is basically used to differentiate mobile devices from one another. It is good to take note of your device’s IMEI number because in case your phone gets lost then there are Android and iOS apps using which you can trace or remotely factory reset it.
IMEI number can also be used by a service provider to block the device from connecting to its network when somebody reports their device lost or stolen.
Changing the IMEI number is considered illegal in many regions since there is no good use of it. But still, thieves change the IMEI number of the devices they stolen to some non-blacklisted IMEI number in order to make it usable again. Therefore you should never share your IMEI number with anyone or else you may end up getting your device cloned.
Much more interesting information that you can find out using IMEI number includes:
-Network and Country of the Device
-Carrier Information etc