Favourites enable you to customize the fields you'd like to see on the Summary tab for all your Crash Reporting errors groups.
When you click an error group in Crash Reporting, you will land on the error group's Summary tab. Yellow stars on the right side of most of the fields indicate that these fields have been saved as a 'favourite' to be viewed on the Summary tab.
If you have not chosen any Summary favourites, the Summary tab will show the default favourites. Navigate to other parts of the error diagnostic information and favourite any of the data to have it appear on the Summary tab and in email notifications.