Press the back button on your controller to access the map. You'd notice that the entire map made up of four big areas/ islands. This achievement requires you to visit each area once. This is pretty much unmissable since you'll be visiting all these four areas en route to completing the game.
If you're in a rush, you can unlock this right after the Night Market mission. Simply grab a bike from your garage, set a waypoint to each area, and go to each place. The achievement will unlock once you enter the area, regardless whether you're driving clockwise or anti-clockwise around the map.