Why do people get put on ECMO?

Could you please explain in simpler terms why a patient might require being put on ECMO? I understand it's a life-saving machine but I'm curious about the specific medical reasons that would necessitate such a drastic measure. For instance, what kind of conditions would lead a doctor to decide that ECMO is the best option to support a patient's failing organs? And how does ECMO work to help these patients recover? Thank you for your insights.