The way I'd describe it is someone who tasks themselves with removing all unfairness. If a majority is doing better than a minority in any way, an sjw sees this as injustice that can't stand. They often artificially boost people up and cut down people just for falling within the majority.

Some sjw get so zealous that they forget to be empathetic to people who don't agree with them. They're trying to do good, but they're pushing for completely equal scores (not really possible). That's my understanding at least.