Far Cry 3 takes place on the Rook Islands, which is an open world and split into two large islands, a northern one and a southern one. The southern island is unlocked until you progress into the story far enough, after which, you will be able to freely explore it.
When you first begin the game, the in-game map doesn’t show too much, but as you progress and do certain things, more will become visible. This includes collectible locations being shown on the map, as well as general areas for the locations of animals. Remember that if you can see it on your map, it’s likely that you can explore it.
Radio Towers are large towers that have been jammed by Vaas and his men, obscuring your visiion of the surrounding area on your map. You are introduced to them in Down in Amanaki Town, where you will be required to climb to the top and disable the jammer. Doing this does two things, making the map clear in the surrounding area and also usually making a weapon or two available in the shop for free. There’s a total of 18 Radio Towers in the game.