The act of using URIs to "refer to" (reference) Web addressable data objects. It's also the act of using the same URI to de-reference the description of a referenced data object; in this case, the representation of the description is negotiated by a Web client and/or Web server. Thus, you can access the description of a data object via data representation formats such as: JSON, XML, (X)HTML, RDF/XML, N3, Turtle, TriX etc.
Note: In proper Web parlance, a data object is referred to as a resource.
Simple example (using DBpedia)
In the Linked Data realm, If you want to make a reference to the Linked Data meme in a blog post, you are better off using the resource URI: http://dbpedia.org/resource/Linked_Data, instead of the Web page URL: http://dbpedia.org/page/Linked_Data, which is the address of a physical document (an information conveying artifact) that at best visually presents the negotiated representation of a resource description.
Why is this valuable?
In the simplest sense, you only have one focal point for referencing (referring to) and de-referencing (retrieving data about) a given Web resource. It protects you from the impact of Web document location changes (amongst many other things).
Remember, a single URI is a conduit into a realm where the identity, access, representation, presentation, and storage of a resource (data object) are completely distinct. It's the mechanism for conducting data across network, machine, operating system, dbms engine, application, and service (API) boundaries. Thus, without "linked data meme" prescribed URI referencing and de-referencing, we are simply back to "business as usual" re. the industry at large, where networks, operating systems, dbms engines, applications, and services (APIs) become the basis for "data lock-in" and silo construction.
Take a second to think about the profound virtues of the ubiquitous Web of Linked Document URLs that we have today, and then apply that thinking to the burgeoning Web of Linked Data URIs, that has just turned corner and heading in everyone's direction at full blast.
Note to "Social Media" players: Who you know isn't the canonical object of sociality. What you are i.e., your description and the data objects it exposes, are real objects of your sociality :-)