To have proper data in place can be very hard when testing applications. Not only you need different data sets to test different scenarios and equivalent classes, but also providing combinations that can be fundamental to catch bugs before they actually going to the production environment.
Additionally, GeneXus applications are commonly data-intensive apps, which means that your tests will not only need a combination of data input but some data configured in your database to let tests run properly.
Mocking database is a technique that allows you to set desired database state (for different tables) in your tests in order to let that data ready for future tests execution, so you can focus on getting the test datasets ready once, and then use it on different test phases regarding of the environments.
This documentation being constructed, since this feature is still not released. Check the roadmap here.