The blue pin appears when you tap your finger on the map view in DJI Go 4. It's just a way ad-hoc to mark a point of interest (where you want to fly your drone to / where your drone might have crashed / a place you need to avoid etc.). The blue pin will move somewhere else if you tap somewhere else. It's useful if you are wanting to zoom in on a particular point on the map, because it will act as a reference point for you to zoom around.
