The IMEI/MEID is a unique number used to identify a phone, device, or modem.
Depending on the device, the IMEI/MEID may be located in the settings, on the back, or near the battery. You can also dial *#06# from most mobile phones to see the IMEI/MEID.
For iOS devices, go to Settings > General > About. Scroll down to see the IMEI/MEID.
For many Android devices, go to Settings (the gear icon) > About phone/device > Status. Scroll down to see the IMEI/MEID.