A citation is essentially a reference to a source of information -- i.e., a book, an article, a webpage, or any other resource, physical or virtual. As such, it is a kind of avatar -- a summary, descriptive stand-in for the object itself.
The purpose of a citation or reference is two-fold. In the first instance, it is an attribution or recognition of the contributions of another researcher, commentator, or author to the work at hand. In the second instance, it is a pathway or, perhaps, a signpost leading others to that contributory source.