"Super Saiyajin" from the anime Dragon Ball Z
the above is the original, japanese, name of the race in the show. After it came to the US and rest of the world, the racename was changed to: "Super Saiyan".
SSJ is therefor a reference to the original "Super Saiyajins".
It is used to describe different forms of the heroes of the show.
Goku is SSJ2, the second level of SSJ.