Ghost of Tsushima is set in a large open world, and there will be a lot of traveling around involved as you try to overcome the Mongol invaders and attempt to save your family from certain death.
To Fast Travel in Ghost of Tsushima, you will need to find Fox Dens. All Fox Dens will have a small fox running around the area near it. When you see the fox, jump down off your horse and begin to follow it. They can walk a pretty meandering route, but will eventually lead you to an Inari Shrine. These Shrines can be tough to reach and may involve some awkward jumps or climbing to get to them. When you reach the Shrine, you can interact with it, and you will receive additional Charm slots or even powerful Charms that you can equip in those slots.
Inari Shrines will also become Fast Travel spots after they have been unlocked, allowing you to get around the map much faster.
When you have them unlocked, you can Fast Travel by opening your map by hitting the Options button, then scroll over one of the Fox Den icons with the cursor. They will look like a small foxes head. Hold the Triangle button and you will fast travel to that area.
The Fox Dens will only become Fast Travel points after you pray at the associated Inari Shrine, so make sure you follow the foxes and get to the Shrines to make traveling around the game much easier. You can also Fast Travel to any major game location that you have discovered and cleared of enemies.