What does -Namespace do in this script: Get-WmiObject WmiMonitorID -Namespace root\wmi

Generally, "namespaces" are like directories ... meaning all WMIs (Windows Management Instrumentations) will be associated to a namespace. This allows us to logically group/associate WMI together with higher level concepts.

From https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.management/get-wmiobject?view=powershell-5.1

The -Namespace parameter:

When used with the Class parameter, the Namespace parameter specifies the WMI repository namespace where the specified WMI class is located. When used with the List parameter, it specifies the namespace from which to gather WMI class information.

The WmiMonitorID is described as such (here --> https://docs.microsoft.com/en-us/windows/desktop/wmicoreprov/wmimonitorid):

The WmiMonitorID WMI class represents the identifying information about a video monitor, such as manufacturer name, year of manufacture, or serial number. The data in this class correspond to data in the Vendor/Product Identification block of Video Input Definition of the Video Electronics Standard Association (VESA) Enhanced Extended Display Identification Data (E-EDID) standard.