Answer: they are referencing that source.
Explanation: A citation is a formal reference to a published or unpublished source that you consulted and obtained information which you applied while writing your research paper or article. It includes the author's name, name of book, year published etc. In addition to this, If the source is central and is fundamental to your work, in a separate sentence or two, people cite their sources by summarizing its importance and main ideas contained. So when you cite a source—writing a short written identification of a previously published work which is used as a source for a text—you are referencing that source.