The Map Browser is where you’ll go to download new single-player maps created by other Far Cry 4 players, including yourself and your friends. There are four basic types: Outpost, which are just like outpost takeovers in the campaign; Extraction, in which you have to reach an extraction point before time expires; Hunt, in which you track down and snag specific wildlife targets; and Assault, a catch-all that includes pretty much anything that involves mixing bad guys with bullets.
Sure, once you get past the maps Ubisoft has already posted, the quality is going to vary a bit from map to map—user-created content always does—but there are plenty of filters and ratings in place to help you sift the awesome from the awful.