Wikipedia will give you a basic overview.

Essentially some feminist wrote a piece about something and was shocked that they were getting mean responses on the internet. As we all know if you spend any amount of time online you'll know that death threats and other horrible things are said by mentally unstable types constantly. But when you're an activist and the cause is all that matters it's a great way to silence your opponents by painting them all as murderous rabble.

Anything less than total support is seen as nigh murder, don't you dare differentiate between calling someone stupid and threatening to rape and murder them.