It is a prayer for freedom for all who need a way out of whatever imprisons us or keeps us in darkness. The 'Key of David' points to Jesus as the one able to open such doors of entrapment.
Well worth reading are Rosemary Hannah's thoughts on this antiphon in a post from last year at Thinking Anglicans. She writes:
There is a terrible reality about David’s mixed desires and ambitions which make him seem astonishingly contemporary.
In that sense, today’s antiphon seems to fit him well – and in fitting him, to fit all those of us who know too well our flawed and dark passions, our divided loyalties and the complexities of our lives. It promises the rescue (by ‘great David’s greater son’) of those in darkness, trapped and ignorant of the paths to escape.
O Key of David and sceptre of the House of Israel;
you open and no one can shut;
you shut and no one can open:
Come and lead the prisoners from the prison house,
those who dwell in darkness and the shadow of death.
Here is the antiphon sung in Latin by the Pontifical North American College Choir in Rome: