There are many types of Windows licenses with varying terms of use. As a general rule of thumb:
OEM keys are shipped with a specific computer and can’t be transferred to another machine. Again, these should be auto-applied from your UEFI/BIOS when reinstalling Windows on a modern boxed PC, but you can also retrieve them manually.
Retail keys are purchased directly from Microsoft, Amazon etc. — these can be transferred to another machine and that process should happen automatically for a digital license, though you can also “uninstall” a Windows key from a given PC.
Those who upgraded to Windows 10 for free from Windows 7 and 8 don’t have a unique Windows 10 key. This can only be transferred to one other machine (not if you upgraded from an OEM key). Free upgrade licenses are a digital entitlement.