dt is the measure of time (in seconds) that elapsed since the last tick.
Let's say you add dt to the x position of a sprite each tick, it will effectively move one pixel to the right every second.
If you want to "mimick" an "every tick" synced at 60fps, you could then add dt*60 to the position, so the sprite would move 60 pixels to the right every second, independently of the current refresh rate.