Addressability is the capacity for an entity to be targeted and found. To be addressable, an entity must be uniquely identifiable, which means that it must be associated with something -- typically an alphanumeric string, although there are other possibilities -- that is not associated with anything else that exists within that system.
A URI (Uniform Resource Identifier) is a unique identifier that makes content addressable on the Internet by uniquely targeting items, such as text, video, images and applications. A URL (Uniform Resource Locator) is a particular type of URI that targets Web pages so that when a browser requests them, they can be found and served to users.
Addressability is an increasing trend: more and more things can be assigned unique identifiers, and if something has a unique identifier, it can be tagged, assigned a URI and targeted over a network. That capacity paves the way for the Internet of Things (IoT), a scenario in which everything -- including people, animals, servers, applications, shampoo bottles, cars, steering wheels, coffee machines, park benches or just about any other random item that comes to mind.-- has a unique identifier and the ability to communicate over the Internet or a similar wide-area network (WAN).