Hello Ljcool, Parallels is a Virtualization software which allows you to run Windows and other OS on your Mac. The term OEM refers to Windows license and it means the OEM Windows license can be used for only one time. Feel free to get back to us, if you need any further assistance.
Permit me, OEM means Original Equipment Manufacturer, an OEM license means that the product is only licensed to run on a a unique machine/piece of hardware and is not transferable (unlike Retail or Volume licenses), however, and referring to your 'only one time' statement, it can be used multiple times (reinstallations) on the same machine.
