Application Access is a feature that allows you to control the access level of other application scopes to your application's data tables. By setting the Accessible from field value to This application scope only, you can restrict the access to your data tables to only your application scope. This means that other application scopes cannot create, read, write, or delete records on your data tables, unless they have explicit permissions through Access Controls or other means.
References:
* Application Access
* [Application scope]
https://developer.servicenow.com/dev.do#!/learn/learning-plans/rome/new_to_servicenow/app_store_learnv2_se
https://docs.servicenow.com/bundle/rome-application-development/page/build/applications/concept/c_ExampleD