An area on the sun where new magnetic flux is erupting.
ON-LINE GLOSSARY OF SOLAR-TERRESTRIAL TERMS of National Geophysics Data Center

An area on the Sun where new magnetic flux is erupting. An EFR is a bipolar magnetic region that first produces a small bipolar plage visible in the chromosphere, which may develop an arch filament system and the initial spots of a sunspot group. An EFR may be isolated from other solar activity or may occur within an active region.