What does it mean to have the autoimmune disease called myasthenia gravis?

Muscle weakness: It causes muscle weakness. It is autoimmune because it has antibodies to your own body, in this case they react with acetylcholine receptors at the neuromuscular junction. Thus the muscles do not get the stimulation to help them function due to the decreased availability of acetylcholine. It is treated with acetylcholinesterase inhibitors that interfer with the the breakdown of acetylcholine.