Linked Data is a movement that sees the web transforming from a web of html, of content, to a web of data. In this web real world Subjects have identifiers and those identifiers are URLs. Resolving a Subject's identifier results in some data. This data in turn is encoded as RDF and has links to other Subjects.
The value in Linked Data is that it exposes Subjects that are important and data about them that other people might find useful. As the data is retrieved using standard protocols and encoded using standard representation formats it can be processed and used by all kinds of different client applications.
Using URIs to name and identify real world subjects allows applications to merge data from different sources that are about the same thing.