it's the common material between Matthew and Luke. and theologians came up with the name Q, which means Quelle/Source in German. there are quite a few books & videos which try to debunk the idea, but once you read it, you know it's related to our Q. you can even use the outlines with your own Bible. I read through everytime I have a dilemma & seems like there's always an answer among the verses; very practical like DJT.