IMEI is an abbreviation for International Mobile Equipment Identity. It's a unique number that all mobile phones have. It's not the same as the serial number, but with IMEI we can retrieve additional information, such as the current value of your phone. It's therefore important that you carefully check that it's the correct IMEI number you are entering. It's 15 digits long and you'll find it in your phone's settings.
Find the IMEI on an iPhone by opening the "Settings" app, selecting "General" and then "About". Scroll down a bit to see the IMEI.
Find IMEI on an Android phone by opening the settings app, scrolling down and selecting "About phone". Scroll down further to find the IMEI number.
Find the IMEI on a Windows phone by starting on the Home screen and swiping from right to left to open the Start menu. Scroll down and select "Settings". Under "System", click "About" and select "More information" to see your IMEI number.