Apud is used in the citation of scholarly works to cite a reference that you do not have direct access to, and are citing second hand; citing "Jones apud Smith" indicates that the original source is Jones, but that the author's source is through Smith's work.
It is also used when referring to the input of a contributing writer or illustrator to a book or paper. For example, an illustration may not appear anywhere but in a paper cited under an author who is not the illustrator. I have seen this usage primarily in works in biology and particularly mycology.
Apud is sometimes abbreviated to ap. It is less common than it once was, and is sometimes replaced by 'in', 'in the work of', 'see in', 'at', or 'according to' in modern works. Apud comes directly from the Latin apud, meaning 'near' or 'alongside'.