have a good command of (something)
To be knowledgeable about or skilled in a particular area or thing. If you don't have a good command of the course material, then you won't do well on the exam.
have a good command of something
to know something well. Bill has a good command of French. Jane has a good command of economic theory.
have a good command of
Have the ability to use or control; have mastery of. For example, She has a remarkably good command of Japanese, or He had a good command of his emotions. [Mid-1600s]