Often a single data type can lead us to other data types. For example, a node has a user (the author) and the user has data associated with it.
A relationship plugin allows this kind of data to be accessed.
An example relationship plugin is provided in plugins/relationships/relcontext_from_simplecontext.inc. It looks at a simplecontext (which we got from an argument) and builds an (artificial) "relcontext" from that.