The so-called "Q" document is a theorized document that biblical scholars believe once existed but no longer exists. They noted several literary similarities between the four Gospels now in the New Testament, and they theorized that a source document that they called "Quelle" (source) must have existed in Biblical times.

So when you see Biblical scholars referring to Q or to a Q document, that's what they are talking about. No one has actually seen this document, it might have burned in the fire that destroyed the Alexandrian Library, but no one knows for sure.