Radio relics are regions emitting radio EMR that are not obviously associated with a particular galaxy. They are assumed to be the left-overs from some past activity. A power law spectrum is common, indicating synchrotron radiation. The regions are typically associated with galaxy clusters. The term fossil radio plasma is used for plasma clouds, the typical case.
One theorized scenario is result of cluster mergers and shock resulting from colliding intracluster media.
Another theorized scenario is clouds previously emitted by galaxies (e.g., the results of jets) similarly colliding. In some cases a galaxy is seen that might be the source of a cloud.