Okay kiddo, so you know how there are these big, furry animals called bison? Well, there's a whole bunch of them that live in a place called Wind Cave National Park.
Now, these bison have been living in Wind Cave for a super long time, way before people started building houses and roads and stuff. They eat grass and roam around in big groups, kind of like a class of students when they go on a field trip.
People who take care of the park, called park rangers, want to make sure that the bison are safe and healthy, so they keep an eye on them. They even count all of the bison every year to make sure the number of them is staying pretty much the same.
Sometimes, when there gets to be too many bison, the park rangers will let some of them go to other places where they can live happily ever after. And sometimes they might bring in some new bison to mix and mingle with the ones already living in Wind Cave.
So basically, the Wind Cave bison herd is just a group of bison who live in a big park and have people looking out for them. Pretty cool, huh?