I think in this context it refers to people who advocate for social justice.
Yes, that's it-- I think SJs is more polite than SJWs (Social Justice Warriors), but I'm guessing about that.
It's a rather confused area of terminology-- there's an older use of "social justice" (note lack of capitalization) which, so far as I know, consisted of advocating for various groups, but didn't include the ideas of privilege and calling out.
Another month, another rationality quotes thread. The rules are: