* -1 is just some math to flip the sign of a number.
100 * -1 = -100 (negative of the original positive number)
-100 * -1 = 100 (positive of the original negative number)
dt is the time in seconds since the last tick - "delta time". 60 * dt means 60 times a second, no matter the framerate. So if you add 60 * dt to an object's X position every tick, it moves right at 60 pixels per second, no matter the framerate.