The letters stand for "Somebody else's problem". And the idea is that if you want to stay out of sight you surround yourself with an SEP field. That way when the party from which you wish to be hidden looks your way, they won't see you really. Well, they'll likely see you but their minds instantly assert "that's somebody else's problem" and they look beyond you or through you, effectively not seeing you.
Works well on cops if you're speeding but this has yet to be scientifically proven.