The blue triangle represents your location on the map. Your approximate location on the map is where the different shades of blue converge inside the triangle. When recording an activity or track on a connected Garmin device, a light blue line of your recorded path will be displayed behind the triangle as you move.
If multiple Garmin devices are connected and recording simultaneously, there may be overlapping lines for each device. You can tap on the map to see a list of the tracks or activities being recorded from each connected device.