The App RW calculates the total wind vectorially combining the natural and true wind with the apparent wind when the boat is moving under sail.
When you are sailing, you are generating a wind on yourself due just to your movement and this is called apparent wind. The apparent wind combine with the real, natural wind, give a total wind, or Resulting Wind, which differs in strength and direction from the natural wind. So when you go sailing, once moving you will have a wind different from that of the weather forecast, and different from the wind you are feeling when still at anchor or dock. The knowledge of this different Resulting Wind is important for example for the choice of strategy, tack and for the choice of the sail to set up..