This is a collectible achievement.
You have to collect 15 masks out of the 55 in the game. The easiest way to find out where the masks are is to buy maps from trading posts at outposts. They cost anywhere between 20,000 and 100,000+.
Once you have the maps the locations of collectibles and other items (specifically for this achievement the Masks of Yalung) for that area should show up. Now you just go to icons for "Masks of Yalung" are located. Unlike most collectibles where it just shows you exactly where the object is you will have a search radius and the mask will be within the area. You can usually find some of the earlier masks without much difficulty as they're usually next to destroyed objects, things of the occult, or just disposed of to get rid of them. BUT if you're having trouble finding one you can unlock the gadget training skill. This will allow you to see where the masks are by looking through your camera. You can scan the area with your camera and whenever you hover over it it'll say to "Set Waypoint" from there just go and get it/destroy it.
Some of the masks (especially the later ones) may be below or above you, if this happens just find a way up/down. Some will also be in tunnels, and passages that might confuse you at first but just search the general area and you'll find a way in.