After completing the introduction and tutorial of Ghost of Tsushima, you will be dropped on the map and free to do pretty much whatever you want. Touring the Hot Springs, Fox Dens and Shinto Shrines is a great way to progress, but don't forget the areas occupied by the Mongols.
Each outpost will increase your legend level and allow you to get your hands on artifacts and many upgrade components. They also allow you to obtain technique points as well as unlock Battle Postures by observing and/or killing Mongol leaders.
Locations occupied by the enemy
Each region of the game represent an act. They are occupied by a number of Mongolian camps. When a place is liberated, the surrounding area on the map is displayed along its points of interest. Liberating places is a great way to explore the map.
The first method to find the location of those camps is to help the natives taken hostage by Mongol groups on the roads. Eliminate the attackers and talk to the victim, they will sometimes give you the position of a camp. Talking to people with a white bubble in towns and villages sometimes has this effect too.
Once all the places in an area are liberated, the area appears in white. This removes the fog of war and all points of interest are displayed. You can't unlock the region in which the act takes place, as the final objective of each act is always to liberate a zone. Note that the last 4 places to liberate from act 1 only appear once act 2 has started, as part of a secondary quest.
Some occupied points can only be freed in the context of a quest.
Hostages
Many of the camps have hostages. Try not to get spotted and kill as many enemies as possible. If the situation ever degenerates, know that a simple blow, a kunai or an arrow on the Mongol marked with a skull will be enough to distract him and make him forget the hostage. You don't need to kill him immediately. It will give you some time before another executioner is appointed. Note that hostages are immune to your attacks, so you can use sticky bombs and explosive arrows nearby to clear groups of enemies.