dt is the time since the last tick.
So at 60 fps dt is around 16 ms.
If your framerate drops so does dt.
If your framerate goes to zero so does dt.
If your frame rate is halved so is dt.
Anything times zero = 0
So if your speed, a value you come up with(in pixels per second), say 60 is multiplied by dt and dt= 0 then the speed will be zero.
If dt was 0.16 then your speed would be 0.96 pixels per tick, 60 pixels per second in this case.
To test set a sprite's x to self.x+(speed*dt)