A red dot
shows your position on the map when the GPS connection is active.
A faint red dot with a large circle around it means no GPS signal is available and Maps is using your cellular network to estimate your position. This is not as accurate as GPS.
The GPS indicator
shows the number of satellite signals available. Each bar represents one satellite.
The bar is yellow when your device is searching for satellites and green when your device has enough information to calculate your position. More green bars indicate a more reliable position.
Your device needs signals from at least four satellites to calculate your starting position. Once it has done this, signals from three satellites may be enough.