The so-called principle of the "occupied field" refers to areas in which treaties have handed law and policy-making powers (competence) to the EU. When this happens, Member states lose their powers (competence) in this area, even if the EU has not yet legislated.
The Lisbon Treaty has a category of competences called shared competences. When the EU legislates in the area it becomes occupied for the member states.
Policy areas taken over by the EU have never been returned to the national level. This can conflict with the principle of subsidiarity, which implies that some matters may be "repatriated" from the supra-national to the national level.