An antigen is an antibody generator which is introduced to the body to provoke a reaction. When appropriate an autoantigen is introduced for specific autoimmune issues.
An autoantigen is an antigen which stimulates autoantibodies in the organism which has produced it.
An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. These antigens should, under normal conditions, not be the target of the immune system, but, due to mainly genetic and environmental factors, the normal immunological tolerance for such an antigen has been lost in these patients.
An autoantigen is something in the body that the immune system attacks but it shouldn't as it should be there. This is the way that autoimmune diseases work. The body fights off or attacks something that should be there. For some reason it doesn't "see" that is "self". Normally, the body will only attack non-self things (antigens) such as bacteria. It will make antibodies that will kill of the bacteria (an antigen). If that same bacteria infects the body again, the body will "remember" it and will be ready. You usually won't even be aware the second time. Here is a partial list of autoimmune diseases: https://labtestsonline.org/understanding/conditions/autoimmune/start/1