A “stack” is essentially the file system and operating system image that your application runs in when you push it to Cloud Foundry.
The stack we’ve used for a few years called ‘cflinuxfs2’ was based upon Ubuntu 14, and that was removed from service on April 30, 2019.
The new ‘cflinuxfs3’ stack is derived from Ubuntu 18, and it is highly recommended that users proactively update their applications to use it for performance and support considerations.