It's a great option for developers to dip their toes into an Azure development environment, with minimal costs:
"The MySQL and Web processes are co-located in the same environment (hence the term in-app) which means storage is shared. It’s cost-effective since there’s no additional cost to use this feature and you only pay for the App Service plan (since resources are shared)"
All the popular environments and web apps are supported:
"It supports PHP or Python applications that use MySQL, such as WordPress, Joomla, Drupal and Django"
Lots more details in the announcement including how a MySQL in-app service can be migrated to a production database solution via a one-click process.