The green dot marks the forecast location you are currently viewing. All forecast information, including the Forecast Tables, Graphs, Daily Briefing, and Wind Maps, relates to this location.
The blue dots represent your other saved forecast locations. They remain visible on the map so you can easily switch between them.
To change the active forecast location, simply tap one of the blue dots and select 'Set as Active' or select a different location from your saved locations list. The selected location will turn green, while the previously selected location will change to blue.
PredictWind uses a location-based forecast system. This means weather information is displayed for the forecast location you have selected, rather than for the centre of the map.