This tag is for questions looking for a single specific reference, or a small number of references. For example, questions of the form "What paper reported the discovery of X?", "Where can I find the original derivation of X?", or "What is the canonical source for X?" are reference requests. For many of these questions, nearly all of the references that could be posted as answers already exist at the time of the question being asked; we don't expect an increase in the number of valid answers as time goes on.
Contrast this with the books tag, which is for questions that ask for a resource explaining a particular topic, but not for a specific resource. The tags reference-request and books are mutually exclusive; they should never both be applied to the same question.