" He [Marty] turned to go, then paused. "Uh, listen," he said, "if you guys ever have kids, and one of them when he's eight years old accidentally sets fire to the living room rug... please go easy on him." / Er... sure," George replied, thinking it one of the strangest requests he had ever heard. / A moment later, he was gone. George and Lorraine stood looking at each other, their hands tightly clasped. "
—From Back to the Future by George Gipe (quote, page 224)

The living room rug was a rug owned by the McFly family in the 1970s.

When Marty McFly was eight years old, he accidentally set fire to it — resulting in his parents, George and Lorraine McFly, coming down hard on him.

When Marty spoke to Lorraine and George, after the Enchantment Under the Sea dance at Hill Valley High School on November 12, 1955, he recommended that if they ever had kids, and one did the aforementioned deed, they go easy on him. Obviously, this kid would go on to be Marty.