Why is it called "Representational State Transfer" ?

The Client references a Web resource using a URL.  A representation of the resource is returned (in this case as an HTML document). The representation (e.g., Boeing747.html) places the client application in a state.  The result of the client traversing a hyperlink in Boeing747.html is another resource accessed.  The new representation places the client application into yet another state.  Thus, the client application changes (transfers) state with each resource representation --> Representation State Transfer