Some Minecraft mods add items that force the game to keep certain chunks loaded. From the casual "Can't you just" I assume the parent meant those. It is however also possible, but much more complicated, to force the game to keep chunks loaded in Vanilla Minecraft. [0]

[0] https://www.youtube.com/watch?v=dx5Wd28AKxQ (the video is a couple years old so the current implementation might be different, but I think the basic principles are still the same).