1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs