I don't know the whole deal with them, but off the top of my head I know it's a very far-right social media site that was fairly mainstream for a while. It got a lot of media coverage after getting hacked, so I guess a lot of people ended up blocking it once they heard of it.
I don't know the full story. They were probably just a bunch of trolls like a lot of the other instances.
The number of instances, not accounts. The instance is the entire server. All of Lemmy.world would be one instance.
Worth noting that there are lots of people who also run personal instances off of their home server. So like they may be the only user in that instance, or have a few friends who use it too. And they would also count as one instance on this chart.