The battle map is one of the main screens in Battle Pirates. It displays all of the areas that your ships have explored, initially only your island base and a small area around it will be visible. As your fleets explore the seas you will be able to see more.
As well as your own island base you will run across island bases belonging to other players, many of these will have a protection 'bubble' over them indicating that the player can not be attacked at the moment due to them either still being within the protection granted at the beginning of the game, or they've been granted a short term protection due to being attacked recently and suffering sufficient damage to their base. Island bases without a bubble can be attacked by your fleets providing you are within 5 levels of them.
You will also find other wonders in the water to be exploited such as mining areas for each type of resource. Each one is defended initially by npc pirates and these must be defeated before the mining area can be claimed. Once claimed the mine will provide a constant bonus to the mines of the matching type back on your own island base. Your fleets may also gather additional resources from the mining area. The level of the mining area will inform you of how much of a bonus to mining it provides, how quickly you can gather additional resources and also how dangerous the npc pirates there will be. Be warned that there are npc pirate bases scattered near the mining areas which will periodically send out ships to try to retake the mining area from you.
You can also find npc pirate ships of various levels scattered around the seas which once defeated can be looted for a finite ammount of resources.