Entity (database) schema is generated based on the base field definitions and configured fields in Drupal 8. When there was a schema change required, update.php had an automated system to fix the entity schema. However, this did not make the entity schema state predictable for module updates. It was not possible to write content updates based on known states of the entity schema. Therefore that magic was removed. If there are modules which need schema updates but they do not apply those updates themselves, the status update page will now inform users about the fact. Because this requires code on the module developer's part, there is no way to fix the issue on the user interface, so the report message (Mismatched entity and/or field definitions
) can only tell you to back out the module install/update:
Developers, the change notice at https://www.drupal.org/node/2554097 outlines the steps you need to take.