Which is mightier the pen or the sword?

It means that Words, particularly the written Word, can be more effective in accomplishing change, i.e. of people's ideas, attitudes, feelings and behavior, rather than using violence. The quote says that great writers (whether in plays, poems, literature, songs, or newspaper reporting) can even outlast an army. Often, despite efforts to suppress them, important ideas will live on in written form, long after the rulers and their armies are gone.