This is a custom dimension of the type "Content Group". In this dimension, static information on a page is collected.
"Bounce Pages" means that only pages are taken into consideration which were the only pages viewed in a visit.
Hint: The "-" classifies all pages that are not assigned to a content group.
Unfortunately, we cannot provide further information on this analysis in the Support Center, as each content group is configured in your Mapp Intelligence system.
For further questions please contact your administrator.