If something conforms to a general pattern, standard, or average, we describe it as normal, but of course that standard can change over time. What's normal today may be "abnormal" in the future.
The word normal comes from a Latin word normalis, which described something made with a carpenter's square. Something built this way would be normed to have angles that were perfectly aligned and fit a general pattern. This meaning eventually gave us the broader sense of fitting a pattern, standard, or average. Doctors like you to be a normal weight for your height, so you may need to grow taller to make your doctor happy.