A cross reference was a note in a document that would direct the reader to relevant information in another part of the same document.
Based on Tanagra results, Lieutenant Commander Data was able to cross reference with the term "Darmok", and establish the two terms were related, but not the narrative behind the relationship, to establish a meaningful response. He later described it as being "analogous to understanding the grammar of a language but none of the vocabulary." (TNG: "Darmok")