A relation refers to the machine-processable description of the relationship between two objects. In Semantic MediaWiki, it's a property of Type:Page that relates one article to another; Type:Page is the default type of a property. A relation is annotated by adding a property to an existing MediaWiki link, using the syntax [[property name::link target]]. A relation always describe a directed relationship between the article with the annotation, and the article that is the target of a link.
Help:Annotation explains properties and annotation in more detail.