As far as I know, #burnsteem25 is an invitation for users to voluntarily burn 25% of the rewards obtained from posts. This is not absolute or mandatory, just voluntary from its users. This "burning" of Steem tokens is intended for a special account that can no longer be used, in this case @null in the hope of reducing the number of tokens in circulation.

In addition to reducing the supply of Steem in circulation, this Steem burning initiative also aims to help maintain or increase the value of Steem tokens by reducing inflation by reducing the number of tokens available. It can be said that this is a form of user support for the long-term Steem economy and building awareness of the importance of token supply management.