URN
In plain terms
A URI that names a resource without giving its location.
Going deeper
A Uniform Resource Name identifies a resource by a persistent name independent of where it resides. It names rather than locates.
Under the hood
Location-independent and meant to be persistent; e.g., ISBN URNs identify books regardless of host.
Examples
urn:isbn:..., persistent identifiers, naming resources, standards IDs.
Think of it like...
Like a person's name, identifying them without saying where they live.