Eczema is a word that means irritated skin. Doctors don't really know why some kids and adults get eczema. When you have eczema, it means your immune system is working too hard. Your immune system usually is good, because it tries to protect you from bad stuff like infections and diseases. For some reason, when you have eczema, your immune system kind of goes crazy. So that makes your skin overreact to something and get all itchy and rashy.