Jesus uses a question to teach the crowd. AT: "You should be able to understand that which is written: 'The stone ... cornerstone.'" (See: [[rc://en/ta/man/translate/figs-rquestion]])
This is the first of three metaphors in a prophecy from the book of Psalms. This one refers to the Messiah as if he were a stone that builders chose not to use, but that God made the most important stone. (See: [[rc://en/ta/man/translate/figs-metaphor]])
"The stone that the builders said was not good enough to use for building." In those days people used stones to build the walls of houses and other buildings.