As punishment for defying Creon's express order not to bury the corpse of her brother Polynices, Antigone has been sentenced to be walled up alive. It's when she's being led away to her dreadful fate that Antigone refers to her tomb as a "vaulted bride-bed." What she means by this is that she's soon to become a bride of death. She will essentially "marry" death by entering into the cave-like structure that will be her final resting place.
Antigone's lines are an example of what's called an apostrophe. This is an exclamatory figure of speech addressed to someone or something not present, often beginning with the vocative "O." Here, Antigone addresses the tomb, turning away from Creon and the Chorus of Theban Elders as she does so. The word "apostrophe" literally means "turning away," and so this crucial moment in the play is fraught with great symbolic significance. This is Antigone's final rejection of Creon's tyrannical rule and all it represents.