It's the Liberation Upon Seeing (Sight) Mantra. "Om Padmo Ushnisha Vimale Hung Phat" in Lantsa script. The Kalachakra symbol that makes it ten fold more effective.
This plaque is to be hung above a door way. Those who pass through it or see the script will not experience the three lower realms, and will be liberated from the fear of falling into the lower realms; will be purified of the three poisons and will be freed from the results of one's karma; will be freed from the fear of remaining in samsara.