The letters stand for "Somebody Else's Problem". An SEP field can be erected on, or projected around a bizarre and unbelievable scene so that the unconscious minds of the observers instantly abdicate responsibility for its existence, assert that it's "somebody else's problem", and therefore don't perceive it at all. The primary example of this was given in the third book Life, the Universe and Everything, when a UFO (A spaceship powered by the Bistromatic drive owned by the fictional character Slartibartfast) landed in the middle of a cricket ground during a match, and the assembled crowd failed to notice it.
The SEP field requires much less energy than a normal invisibility field, due to the natural propensity of humans to see things as Somebody Else's Problem.
See also: bystander effect