In Antigone, as well as in other Greek literature, Teiresias is a prophet, sent by the gods to reveal truth to those who will hear him. Another name for a prophet is a "seer," and, ironically, this seer is literally blind though he is able to figuratively "see" the truth as well as the future. That's what prohets do--they tell the truth and foretell the future--usually a future of doom and destruction.
In this story, Teiresias functions as an emissary of truth. Oedipus the king has asked the Oracles to reveal the cause of his country's literal dying--children are stillborn, crops are withering, animals are dropping in the fields. Teiresias arrives to, as requested, reveal the cause of the curse. He speaks the truth, and Oedipus is named as the root and cause of the city's curse. Instead of taking him seriously, as a representative of the gods who has come at his request, Oedipus does the same thing he did the last time he visited with an Oracle--he dismisses the idea as ridiculous and follows his own thinking and logic. Even worse, in this case, he treats the old, blind prophet abominably. Teiresias is scorned and mocked and even physically abused by the arrogant Oedipus. The prophet's role is to speak the truth, to help the blind see, so to speak; instead, it is Oedipus who is blind and refuses to "see" the truth set before him. Once his story has been made known, of course, Oedipus blinds himself, another twist of irony in this play.