To specialize is to narrow down your focus, in your studies or profession, to a specific field. If you love playing the accordion, perhaps you should specialize in polka music.
If there’s a subject you think is special, specialize in it! For example, doctors start to specialize in medical school, when they study something specific such as podiatry (feet) or cardiology (hearts). Other professionals also specialize in smaller parts of larger fields, like lawyers who specialize in corporate law, or writers who specialize in TV comedies.