It stands for "hard fork 19". It's the latest update to Steemit. One aspect of it that has changed is it's narrowed the distance between whales (the largest accounts) and minnows (the smallest accounts) in terms of voting power. Meaning those with very powerful accounts are a little less powerful, and those with very weak accounts are much more powerful.

People are freaking out because we are seeing HUGE payouts in the flash of an eye.