The postdoctoral phase is the stage between the conferral of your PhD and an appointment to an (assistant) professorship. The goal of this phase is to gain the scholarly qualifications and experience necessary to apply for a professorship.
There is no set job description for postdoctoral work as postdocs carry out tasks in accordance to their subject field and the individual stage in their career. Nonetheless, all postdocs perform their own research and, if possible, give courses, supervise students and PhD candidates, and apply for external funds. If a habilitation is required for a professorship, this should completed during the postdoctoral phase.
Working and performing research at a different university is a vital step in gaining varied and broad experience in a given area of study. The postdoctoral phase is the ideal time to move to another university or to go abroad for a research stay.