What is it when you have pain in the left shoulder blade?

Muscle strain. Usually it's muscle strain. But it can also be a pinched nerve, or possible a mass. See an orthopedic doctor, who wil start with xrays.
Could be many things. Without more history and physical exam, pain in the shoulder blade area could be anything from basic muscle strain or trigger points, to heartburn, to serious problems like heart pain from angina, lung disease, or pancreatitis. Need more info.