HF20 is short for Hardfork 20 (also called Steem Velocity), meaning the 20th hardfork done to the Steem blockchain. A hardfork in itself is a type of update, just like when you update iOS or any other software on your electronic device. Anyway, let's get into the details about what exactly changed on Steem with the HF20 update that was published a few days ago!
This is a very long answer though, but I feel like it is best to go into detail about the different parts to make it as understandable as possible.
1) Unlimited editing. You can now edit post and comments at any time, even after they have received their payout. Keep in mind that a previous version is of course stored on the blockchain, so it will not be "deleted forever" if you decide to remove all the text from a post or comment.
2) Changing "bandwidth" to Resource Credits. You are probably aware of this already, since it caused so much damage at the launch! What happened is that the old bandwidth system was very unfair when it came to allocating resourced to people, so the new hardfork changed it to a completely new system called Resource Credits (RCs) that is supposedly a lot better in the long-term. Just like with bandwidth, you now spend RCs for every action you take on the blockchain, including voting, posting, voting for witnesses, and commenting.
3) Changing the cost for signing up new users. Instead of requiring either a delegation or Steem to be powered up to the new account, each account now cost 3 STEEM to be created. These 3 STEEM are then burned, but new users can still perform actions worth these 3 STEEM on the blockchain, since they will still get the RCs from them.
4) Dust Vote Threshold Changes. To summarize it; you can now vote with any amount of SP, even very low amounts, but there is still a dust threshhold, and a new dust threshhold added to every single vote. It makes it less profitable to vote extremely low on a huge amount of posts.
5) Curation window changed. Previously you used to have to wait 30 minutes to get maximum curation rewards for voting on a new post, and if you voted before that a certain % of the curation reward was instead awarded to the author of the post/comment that received the upvote. This has now been reduced to 15 minutes, so you should upvote on any comment or post that is over 15 minutes old if you want to get all the curation rewards for yourself. Another change is that the % that was given to the author before the hardfork now goes back tot he reward pool, so the author has nothing to gain from getting early votes.
In other words, if you want the best possible reward for both yourself and the author, vote after exactly 15 minutes.
6) Buy and sell order now expire on the internal market. They used to last forever, but after HF20 these buy and sell orders now expire after 28 days. This is very common for exchanges, so it's a good update in my mind. It also limits the amount of low-bid "spam" we see on the market.
7) Parameter changes for the witnesses. The witnseese have had some parameter changes to their servers, but this is not something to worry about unless you run a witness node.
8) Removal of the 20 second comment restriction. You can now post a comment on every single block, meaning that it's in reality a 3 second restriction on commenting instead of a 20 second restriction. This is pretty great for dapps such as DPoll.xyz and other dapps that uses a lot of comments.
9) Fixing bugs and exploits. Various problems with bugs and exploits have been fixed, including stuff like the infamous double voting exploit where a users would spend their voting power, then delegate their SP away to vote again.
10) Decreased delegation cooldown. When you removed a delegation, it used to take 7 days to get your Steem Power back and useable, but this was changed to only 5 days. The reason is that since they have fixed the exploit mentioned above, there's no need to keep it at 7 days.
11) Changes to upvote lockout period. You could previously only flag posts during the last 12 hours before a payout, and not upvote it, but this has now been changed. Now we can still both upvote and downvote during the last 12 hours before the payout, but the strenght of these votes/flags are gradually reduced to being worth 0 % once the payout is reached. So at 6 hours before payout, a vote is only worth 50 % of the normal value.
12) Changes to the SBD print rate. SBD will now be printed until the debt ratio reaches 9% of the STEEM market cap, and it will then gradually change to STEEM until it hits 100 % STEEM and 0 % SBD at 10 % debt. Previous values was 2 % to 5 %, so this means that you can expect to see some more SBD payouts again now.
13) Beneficiaries now get paid using the same payment method as the author. This is amazing news for developers who run a dapp, since they can now get paid some liquid STEEM or SBD in addition to the Steem Power they used to get.
I think these are the 13 big changes we have seen with HF20, but there are some minor changes such as fixing typos and that kind of stuff as well. I hope you learned something from this!
All the updates can be read about in greater details over at @steemitblog's post about it: https://steemit.com/steem/@steemitblog/steem-velocity-hardfork-hardfork-20
This is a bit more technical as well, so you might enjoy that if you are into that type of stuff.