The map overview shows all the locations that you can go to, along with any party locations available to you.
When you click on the map icon (with the tooltip "Missions and parties") on the action bar, you are taken to a location page. To see the map overview, you have to click the map icon at the top left of the page, next to the ? icon (help text).
This is the map overview with all locations uncovered and labelled.
Help text Edit
If you click on the ? icon, you see a pop-up with the following text:
The great wide world. Well, at least Los Vengeles and environs! The areas you can visit are displayed in colour, the ones you have no access to are in grey.
If you want to visit another area, click on the area in the map. Travelling can be so easy...