It's one of Lama Zopa's "Powerful Mantras for the Time of Death", esp. for preventing/liberating from lower rebirths and of general benefit to those on their death bed. You might want to search Lama Zopa's teachings at the FPMT (Foundation for the Preservation of the Mahayana Tradition) Web site: http://www.fpmt.org/
There's a also tradition (I could try to get a precise reference if desired) which holds that saying it 7 times per day will prevent you from being reborn in the lower realms.
It's also I believe a general purification mantra, and as the prev. poster mentioned, is often placed over entrances, lintels, etc.