SoV stands for 'Strength of Victory'. Its also sometimes referred to as margin of victory. If a player picks a team that beats their opponent by 10 points, then their SoV for that game is 10. If the picked team loses, the SoV is -10. The SoV shown on standings pages is cumulative, that is, it's the sum of all SoVs for each game. For example, if a player picks the winner of the first game and the team wins by 17, and picks the loser of the 2nd game that loses by 5, their total SoV across those games is 17-5=12.
If your league is against the spread, the SoV takes this into account. For example, if Denver is favored by 6 over the Raiders, and wins by 14, the SoV is 8.