So, imagine you're playing in a sandbox - you know, the big box filled with sand at the playground. Now, imagine that instead of just one sandbox, there's a whole bunch of them, all piled up together! That's what sandhills in Nebraska look like - a really big area filled with lots and lots of sandy hills.
The sandhills were formed a long, long time ago when a big sea covered the area that's now Nebraska. As the sea went away, it left behind lots of sand and dirt, and then strong winds blew that sand and dirt around until it formed hills called sand dunes. Those dunes just kept building up over thousands and thousands of years, until finally they formed huge sandhills that are now a big part of Nebraska's landscape.
The sandhills have a lot of different plant and animal life living there, and people also use the area for things like ranching and farming. It's a pretty cool place to visit and explore, especially if you like sand and adventure!