It not generally used in reallife. It is originally come from Japanese Animation & Games, is generally come from games. In real life school ranking mark only A, B, C is credit marks, this will feel not enough for high ranking. In Games, is freely to input A+, A-, B+, B- rank system, but games generally put on SSS, SS, S, A, B, C, D, E, F, make players feel more happy on their result.
It means Super or Superior or alike surprise. Superlative classification. Super x3, Super x2, is weird to put Sx2, Sx3, so it is SSS, SS.
Some of games you can see R, SR, SSR, UR. R is meaning "Rare", mostly in card games. That means you get a rare card. SR is Super Rare. SSR is Specially Super Rare sometimes. UR is Ultimate Rare.
Rare[R]Super Rare[SR] Card Sample , Ultimate Rare[UR] Card sample
I remember I learned about this game s-ranking system, but I don't remember who is the author of making this system, sorry. I remember have origin author who make this s-rank, but not remember the name, this is minor title in history.
Such as statement "The atom bomb S-Ranked Hiroshima" from Urban Dictionary: S-Rank. And, SS-Rank is by The early rank system of 1926, insignia of the Schutzstaffel. Early military.
Information is here:
Giant Bomb: S-Rank,