DCF77 is a longwave time signal radio station. The callsign stands for D=Deutsche (German), C=long wave signal, F=Frankfurt, 77=frequency: 77.5 kHz. It is run by The Physikalisch-Technische Bundesanstalt.
DCF77 radio signals (set by an atomic clock) are broadcast on 77.5 kHz from Mainflingen, Germany, about 25 km south-east of Frankfurt. The signal can be received from a distance of 2000 km. There are 2 transmitters so it is rarely off the air.
Radio clocks are popular in this region and use DCF77's signal to set their time automatically.