The Deviationline is a line that is drawn from the plane location (fixed by GPS or Flightsimulator) to the middle of the map location. Normally these two locations are the same except when you reposition the map by dragging it manually. Is the distance between the plane location and the map location bigger than 5Nm a diviationline is drawn with a coarse marker on the end showing the coarse (true north) and the distance to this location. This is a very handy aid when planning a flight or when direct deviation to a new location during the flight is necessary.
