The word “voluntary” can be used for the title of a piece of music. The title was often used by English composers in the late Renaissance or Baroque periods for a piece of organ music that was free in style, i.e. it did not have to be composed in a strict form such as sonata form or a fugue. It was meant to sound as if it was being improvised (the word voluntary in general means “free”, i.e. not “forced to do something”).