A superior generally has the power to approve or deny requests from subordinates, within the scope of the relevant organization.
The superior may control the careers of subordinates; for instance, they may have the authority to give raises or promotions.
A religious superior is the person to whom a cleric is immediately responsible under canon law.
For monks, it would be the abbot (or the abbess for nuns); for friars, it would be the prior, or, for Franciscans, the guardian (custos), for Minims, the corrector; for diocesan priests, it would be the local bishop.
The priest in charge a mission sui iuris is called an ecclesiastical superior.