Rockstar has put incredible effort into building a beautifully detailed world full of sights worth seeing. However eventually you'll find yourself traveling through the same locations repeatedly throughout your journeys.
Travel can take a long time. This isn't GTA 5 where you have high-performance vehicles or even aircraft to speed you on your way. When crossing the vast distances of the frontier in Red Dead Redemption 2, you have the option of putting your horse on autopilot for a hands-off experience.
Whether you want to take in the scenery without having to control the steed, or you'd like to pop off to make a cup of coffee or something during the travel time, this is a great feature to have. You can set a destination on your map screen and have the horse travel to it by following roads and paths. Instead of just beelining it to the marker through the wilderness, this generally prevents pathfinding issues and threats from bandits or wildlife.
When you are at the map screen, you can set a custom waypoint by pressing X/A. The map should then display a path that follows roads for the most part, which is what the horse would follow if you'd put it on autopilot. Now exit map view and start riding. Spur your horse with X/A until you reach your desired travel speed, then keep the spur button pressed to hold speed.
On PC this mode can be activated by pressing the down key and holding W, and then holding B until the bar fills up - depending on your keybindings, of course.
Then, you need to hold down the button used to switch camera perspectives, which is the trackpad or "View". Keep it pressed until the little circle indicating the switch to the cinematic camera is filled. This way you'll get a wide angle view of the world around you without any HUD elements clogging it up, and the horse will proceed to its destination on its own.
You can speed up or slow down your horse without coming out of autopilot, and you can also move the camera around at will. Simply move the analog stick for movement to regain control, or press the camera switching button again.