URI
In plain terms
A string that identifies a resource, by name or location.
Going deeper
A Uniform Resource Identifier is the general term for strings that name or locate resources; URLs and URNs are types of URIs. It's the umbrella concept.
Under the hood
URLs specify location; URNs specify name; all URIs follow a common syntax.
Examples
URLs, URNs, resource identification, REST endpoints, web addresses.
Think of it like...
Like the general concept of an address, covering both street locations and proper names.