There are a total of 34 Outposts scattered across the islands. The Outposts are the strongholds for Vass's/Hoyt's men and control certain areas of the map (shown in red) and are represented by a Flag icon. Taking over an Outpost clears the surrounding area of the enemy AI.
To clear the an Outpost, you must dispose of all the enemy AI inside and around it. This can vary from 3 guys, to 5 and some guard dogs. Your best bet is to use a stealth like tactic, as if they sound the alarm (which you can disable) more guards will come to assist.