The Standard Scratch Score (SSS) is a measure of the playing difficulty of a golf course for a scratch golfer under normal midseason course and weather conditions.
It's a method of equating courses, so that a handicap at one course, say Par 72 SSS 70 would be equivalent to one at another course - say Par 68 SSS 71.
It's the SSS that handicap is basically measured against - though it's 'adjusted' in any competition (the competition stndard scratch - CSS) supposedly to allow for conditions on any particular day.
In the case of the recent US Open where, i believe, at least 1 Par 5s was re-badged as a Par 4, the SSS wouldn't have changed (had there been one!).