Sacking out is a way to help horses learn not to be scared of things that might be scary or uncomfortable for them. It's like helping a little kid not be scared of the dark.
To "sack out" a horse, someone will gently touch different parts of the horse's body with something that might be scary or uncomfortable, like a plastic bag or a piece of rope. At first, the horse might get scared or try to move away from the scary thing. But over time, the horse will learn that the scary thing isn't actually going to hurt them.
This helps the horse become more relaxed and confident around new and scary things, which can make them safer and easier to handle. It's kind of like teaching a horse to be brave!