Repetitive Transcranial Magnetic Stimulation is a procedure that uses magnetic fields to stimulate nerve cells in the regions of your brain involved in mood control. rTMS is FDA approved and addresses a variety of neurological and psychiatric disorders including depression, migraines, Parkinson's disease, dystonia, tinnitus and auditory hallucinations.
From the Mayo Clinic website:
"During a TMS session, an electromagnetic coil (crown) is placed against your scalp near your forehead. The electromagnet painlessly delivers a magnetic pulse that stimulates nerve cells in the region of your brain involved in mood control and depression. And it may activate regions of the brain that have decreased activity in people with depression."
Though the biology of why rTMS works isn't completely understood, the stimulation appears to affect how this part of the brain is working, which in turn seems to ease depression symptoms and improve mood.
Treatment for depression involves delivering repetitive magnetic pulses, so it's called repetitive TMS or rTMS.