The negative slope means that you move downhill as you go from left to right. Think of it as a mountain slope and the point on the line slides downhill. The direction of movement is from left to right. To be technical, in this case you will have a rise of -1 indicating you go down 1. At the same time, you go to the right 1 unit
rise/run = -1/1 = -1 = slope
So that's a more technical breakdown of the slope of -1
