Traditionally, a Metropolitan was the chief bishop of a Roman province or Metropolis. In the case of the OCA, which is autocephalous, the Metropolitan is the highest-ranking bishop (or official of any kind) in the Church. However, the authority of a Metropolitan (like that of a Patriarch or Pope, not of Rome) is neither absolute nor particularly concerned with temporal matters. It's an honor, but as I understand it it differs from the work of a regular bishop mostly in that there's a lot more organizational and bureaucratic skill involved. In return, you get really pretty clothes and a megaphone for most of your utterances.

That's not perfect, but I think it's a pretty decent summary.