what does the j in ssj mean?

It's Japanese for Super Saiyan. They say Super Saijin or something close to that. The English shortcut is SS and Japanese is SSJ but a lot of people like the Japanese other than English shortcuts for those.

SSJ stands for super saiyan jin
Idk why it says jin, but I'm sure jin is something unrelated or important and most certainly isn't something other than ss super saiyan, that is just a shorter version.

Jin means a type of people I believe the ss is elf explanitory

The 'jin' is used to indicate a people in 'Saiyan-jin'. So, if you were from America you are America-jin. In this way you indicate you are of the America people. It would be the equivalent of saying American. So, Saiyan-jin is just a formal way of saying that it is somebody that is Saiyan. Or for another way of explaining it. 'Saiyan-jin' refers directly to the Saiyan people as a whole. And when referring to an individual you are indicating that that individual is in fact born in the bloodline of that people/ethnicity.

Ss is the american version meaning super saiyan. But in japan they call it super sajin ,ssj

SSJ is short for Super Saiyajin. Saiyajin is the original japanese pronunciation of a race of humanoids with a tail that resembles a monkey tail and can transform into great apes with immense strength. Super Saiyajin was a legend or myth that a saiyajin can obtain so much power that they radiate golden ki and their hair becomes dyed in their ki. The reason their eyes become green is never explained, since the original concept color was red to represent the rage of the super saiyajin.

Off-Topic and History Lesson:
For those of you who have heard Sayian and don't know where it came from but just say it as if it's simply semantics.
Sayian is the shortcut cantonese pronunciation of the race Goku that belonged to. This is partly to be explained by history in that most early adopters of japanese anime internationally was from Hong Kong and South Korea and eventually brought to USA and UK by asian immigrants, travelers or some nosy white dudes. At the time Sayian was the HK dubbed pronunciation that was much more prevalent in America along the West Coast. Later when cartoon network picked up DBZ they simply decided to just use Sayian instead of the original Saiyajin for probably a lot of good reasons. It's easier to say and remember. In fact there use to be a forum of people arguing if Sayian was a proper pronunciation and eventually most just agreed it was for the better. Have fun!