The axis on which Earth rotates is at an angle relative to Earth's orbit around the sun. One of the consequences of this is that around June, if you travel close enough to the North Pole, you'll experience days of continual sunshine. From the ground, the sun will appear to circle around you without ever setting below the horizon. A similar thing happens in December, near the South Pole.
Flat Earthers say the latter thing, the continuous daylight in the South Pole, is impossible. Their model of the world has the Arctic in the center, and the Antarctic exists as an ice wall encircling the restbof the world. Continuous daylight in the Arctic is the result of the sun making a tighter orbit around the center of the world around June, but there isn't a wider orbit around December that would give the entire Antarctic continuous daylight.
The Final Experiment is going on now. A bunch of flat earthers, and a bunch of skeptics as well, have been invited to an Antarctic research station to personally witness the continuous daylight and record the sun with whatever equipment they saw fit to bring with them.