Here is a really good explanation of OEM from "A. user" (Riccardo):
OEM means Original Equipment Manufacturer. This means that when you purchased your computer your computer manufacturer put on the OEM copy of Windows 10. Via Microsoft licensing rights, OEM registers to the motherboard and when you change that piece of hardware you need to purchase a new retail version of Microsoft Windows 10.
A really good explanation is:
If you upgraded from a retail copy of Windows 7, Windows 8 or 8.1, the Windows 10 license carries the retail rights from which it was derived – can be transferred. But under Microsoft’s rules, you are only entitled to a one time transfer.
If you upgraded from an OEM Windows 7, Windows 8 or 8.1 license, these are licenses that come preinstalled on a new computer from a manufacturer, and then your Windows 10 license maintains the OEM rights – cannot be transferred.
If you have a full retail copy of Windows 10, you can transfer it as many times as you want.
I'm guessing this came with the PC you bought, which the Windows key is now embedded in the motherboard (so if you re-load Windows 10 it will automatically pick up this code).
If this helps please mark as correct answer.
