This parameter refers to the Serial Number generated during an installation of Windows, which can differ from the Product Key associated with a Windows license.
You can find out how to locate the Product Key associated with the installed license here: https://support.microsoft.com/en-us/help/10749/windows-10-find-product-key
The Serial Number, on the other hand, should be unique to the individual computer that Windows is installed on, as opposed to the Product Key which may be shared among many computers when a volume license is used. This is most commonly used by businesses for deploying a large number of computers for an office or other facility, but can also occur when OEMs (original equipment manufacturers) use a volume license to activate computers assembled for sale.
On some occasions, such as after installing a store-bought version of Windows yourself, both the Product Key and Serial Number will be the same.
This Serial Number may be requested by Microsoft Support when contacting them for assistance, so it can be quite convenient to be able to easily locate it in Speccy.
The Serial Number will not be saved when a Speccy Snapshot is saved or published, to prevent this information from being unintentionally shared.