SR is short for Synchrotron radiation. It's emitted when a charged particle encounters a strong magnetic field. The particle then accelerates along a spiral path following the magnetic field and emitting radio waves in the process. The result is a distinct radio signature revealing the strength of the magnetic field.
SR is produced by the spiraling electrons along Birkeland currents. Non-relativistic moving particles produce radiation called cyclotron emission. Relativistic moving particles produce synchrotron emission.
The radiation is generated by astronomical objects, around which relativistic electrons spiral (and hence change velocity) through magnetic fields. The radiation produced has a characteristic polarization, and the frequencies generated can range over the entire electromagnetic spectrum, which is then called continuum radiation.