Upon reading Strobel's book, he has a section written about "Q". Here is a section:

"In addition to the four gospels, scholars often refer to what they call Q, which stands for the German word Quelle , or "source". Because of similarities in language and context, it has traditionally been assumed that Matthew and Luke drew upon Mark's earlier gospel in writing their own. In addition, scholars have said that Matthew and Luke also incorporated some material from this mysterious Q, material that is absent from Mark."