If you are using one of the following device: iPhone 7, iPhone 7 Plus, iPhone 6s, iPhone 6s Plus you may find the IMEI number on the SIM tray.

The IMEI Number should pop up on the screen.

Type in the following secret *#06# and tap Send button.

Here you should find such information as Serial Number, IMEI/MEID, and ICCID.

At the very beginning from the Home Screen find and select Settings app.

Read the this article to get to know all of them. You may find these numbers by using a couple of alternative methods like in the Settings menu, on the physical device, in iTunes, or on the original packaging. Number is used by the GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing the network in that country. The IMEI is only used for identifying the device and has no permanent or semi-permanent relation to the subscriber. The IMEI Number (International Mobile Equipment Identity) is a unique number to identify GSM, WCDMA, and iDEN mobile phones, as well as some satellite phones. The serial number consist of character string that includes letter, numerical digits or even typographical symbols. The Serial Number (also called serial code) is a uniqe identifier assigned incrementally or sequentially to an iOS device.

Look up apple serial number