Its only function is to make darkness and light a valid mechanic for the default minetest_game. In other voxel games, darkness is dangerous because it spawns monsters. In Minetest Game, darkness just makes it more likely for you to walk into a tree.
This mod changes that in a very direct fashion: you are damaged by darkness, the darker the more damage. So craft those torches!
If you stand in a node with light level 7 or less, you slowly lose "sanity", represented by a hud bar with eyes. The darker it is, the more sanity you lose per second. When you run out of sanity, you get damaged instead!
Stand in bright light to replenish sanity. Sunlight is best, but directly on top of a torch should work, too.