Irradiated Areas
Irradiated Areas are select zones in Rust that can harm you if you stay in them too long. These areas are filled with Radiation -- hence the name -- which will increase the Rads bar in your Status, and may even give you the Radiation Condition.
Nothing will happen to you until you reach 500 Rads. When that happens, you will start losing health. To reduce your Rads, stay out of irradiated areas long enough or consume Anti-Radiation Pills or food that reduces Radiation. Another good option is wearing Radiation Suits before going into irradiated areas. You will need to find the suits or the blueprints for the suits before you can craft them, however.
Why would you want to go into an irradiated area? These places usually have a ton of very valuable items that you can't normally find in the wild parts of Rust Island. Some items can only be found in Airdrops or these irradiated areas, and it's a safer and easier to find these areas than wait for airdrops that can attract any and everyone nearby.