Once an EU law is passed, it can be necessary to update it to reflect developments in a particular sector or to ensure that it is implemented properly. Parliament and Council can authorise the Commission to adopt delegated or implementing acts, respectively, in order to do this. Primary responsibility for implementing EU law lies with EU countries. However, in areas where uniform conditions for implementation are needed (taxation, agriculture, the internal market, health and food safety, etc.), the Commission (or exceptionally the Council) adopts an implementing act.
Definition from https://ec.europa.eu/cefdigital/wiki/display/CEFDIGITAL/CEF+Glossary
