"This is heavy" was an expression used frequently by Marty McFly to mean a situation that was very ponderous or serious. Since it was a slang of the 1960s which passed into the 1980s, its meaning was not clear while in 1955; when Emmett Brown overheard it, he wondered at first what weight had to do with the problem. Later, he wanted to know if there was a problem with the Earth's gravitational pull in the future. After Doc first demonstrated how the time machine worked, Marty used a similar phrase, "This is heavy-duty", which he restated as "This is great."