It will contain numerous bug fixes as well as further refinements to the Joomla! core and will be available to all existing customers whose websites are up to date. It will be applied via their SLA (automatically or manually) or via an estimate provided closer to release. Nothing will break during this update.
But the biggest announcement was confirmation that Joomla! 4.0 is on the way.
The Joomla! 4.0 update is a backward compatibility breaking update, and there will be a significant amount of work involved in updating your site to it. But don’t worry - If you have a Joomla! 3.X website you will still receive patches and security updates for two years following the release of version 4.0, the date of which is yet to be announced.