Think of HF-20 like a software update, witnesses like servers of steem, they updated the software at the servers but the software was buggy, then they released patches to fix the bugs. Now, it's running smoothly.

HF-20 stands for Hardfork 20 by the way also code-named velocity. The blue banner on top of the site when you first open it will take you to a post explaining in detail about it.