It represents the slope. Slope is how steep the graph is. In this case, when you divde it, it equals 77. That means that when you graph the line, it goes up 77 units (+77 on the y axis) every time it goes right one unit (+1 on the x axis). It also means that it goes down 77 units (-77 on the y axis) every time it goes left one unit (-1 on the x axis).