Jocasta offers proof to Oedipus that an oracle had once prophesied that her husband Laius would die by the hand of his own son. Out of fear of death, Laius had a shepherd take the boy (with Jocastas consent) to leave him to die on a hillside pinned by the ankles. Laius, in turn, was killed by robbers not his son, and the oracle was proven incorrect.
Oedipus is troubled because she says that Laius was killed at a place where three roads meet, something that reminds Oedipus of an occurrence in his past.
The only eyewitness to Laius's death was a herdsman.
Oedipus had fled Corinth because when in doubt of his parentage he'd gone to an oracle who'd told him that he would in the future kill his father and marry his mother. He was so frightened he ran away.
He insisted that Jocasta send for the herdsman.