It's an initiative where you set 25% beneficiaries to @null which would burn 25% of your post payout. This is being done to help maintain the value of Steem. There are a few links which will further aid to your understanding:

Why are we burning Steem? What is Proof of Burn and what changes can burning steem bring? || #burnsteem25

Do we need to Burn STEEM ?

Hope this helps :')