SOR stands for Sum of Ranks. It is a hypothetical tie breaker. In a tournament where you play 5d (5 dan) 4d and 3d your SOR is 12. If kyu ranks appear it gets slightly more complicated :) Try using 7k as -6 and 1k as 0.
SOR can also stand for Sum Of Ratings where one performs the same calculation as above but uses ratings
This tiebreaker is trying to assess who had the toughest opponents in obtaining their score. It could be used in order to remove the veil of equality imposed by the McMahon bar, by implicitly recognizing that some players above the bar are deemed to be stronger than others.
Tapir: Before using the opponents ranks / ratings as a tie breaker, it should be considered to use the ranks / ratings of the tied players themselves for breaking ties. More on that: Previous performance
This tiebreaker relies on the previous performance of the opponents faced. It does not rely on their actual performance in the tournament. As such it cannot take into account issues such as an opponent having (secretly) improved 500 GoR since their last tournament nor on a 2800 player being as sick as a dog during an event.