Because drama is, in ancient terms, pure mimesis with no diegesis, events occuring offstage must be reported. Where, in a novel, the backstory or events not seen by a viewpoint character can be reported by a narrator, in drama, they must be described in speech.
Certain standard mechanisms are often used for such narration in classical drama. First, messengers often report off-stage action. Second, the chorus can function as a sort of pseudo-narrator. Third, characters can explain their actions to the chorus after the actions have been completed offstage.
As you read through Medea, look closely at the use of the Nurse in narrating offstage (and prior) events, and also at speeches in which Medea explains her actions to the chorus.