What are Quick Filters?
Quick Filters allow users to filter displayed data according to any JQL (Jira Query Language) statement. This gives the user great flexibility with what is displayed in the window.
Information about the JQL as the advanced form of searching, query types and more can be found in the official Atlassian Jira documentation.
By default, there are a few Quick Filters available on the app's header. These can of course be deleted. Among these filters you can find: My tasks (tasks Assigned to the User), Unresolved (tasks in the status Open and Reopened), Updated <5d (tasks updated within last 5 days).
When an issue is a parent and does not fit the JQL query while at the same time its children do fit the JQL filter the parent task will be shaded out - just as when using the JQL search box.