My doc says I've got autoimmune diseases. What does this mean?

Body attacks itself. An autoimmune disorder occurs when the immune system that normally fights off invading germs gets confused and sees some of your normal body cells as "germs" and starts to attack and destroy them. The type and severity of the autoimmune disease depends upon which cells are attacked.
Self destruction. When the immune system sees someone own cells or certain tissue of their body as foreign and mount an attack against them and start destroying them .Let's say for example if immune system sees someone own red blood cells as something foreign, it will start destroying them, causing this person to develop hemolytic anemia, in some came cases the joints can be the target of destruction.