O Key of David, and scepter of the house of Israel, who opens and no man shuts, who shuts and no man opens: come, and bring forth the captive from his prison, he who sits in darkness and in the shadow of death.
What "captive" are we speaking of here, asking Christ to come set free? Ourselves. Each one of us is that captive, trapped "in darkness and in the shadow of death" by our sins. Only Christ has the key, is the Key, to free us from our prisons.
Today's antiphon sounds reminiscent of Matthew 16:19, wherein Christ tells Peter that He gives him the "keys to the kingdom of heaven," and bestows upon him (and a few chapters later, the other apostles) the authority to bind and loose, open and shut. In today's verse, we proclaim belief that Christ comes bearing these powers, in order that He (and down through the ages, priests and bishops to whom the ability to cooperate with His authority has been passed down) may loose us from the sins that bind us.
O come Thou Key of David come
And open wide our heavenly home;
Make safe the way that leads on high,
And close the path to misery.
Rejoice, rejoice! Emmanuel shall come to thee, O Israel.