An outpost is marked on your map as a red flag. These outposts are guarded by pirates and occasionally dogs as well. You can go about liberating them in any way you see fit. If you want to rush in with an assault rifle or you would rather take a sniper rifle and eliminate the opposition from a distance...that's up to you.
After you have liberated 3 outposts from pirate control, you will pop this trophy. Note that those outposts that are liberated will be outposts that you can fast travel to. So liberate as many as you can!