Community admins and group moderator are able to assign a parent topics to a group. Meaning, when a user goes to the topics page, the related groups to that topic will be promoted in the topic page as well. So, people with the same interest can gather in a group related to their favotire topic(s).
This is hoe you can add the parent topic to a group:
Go to the Group page> click on the (...) button on top right and next to the "Moderate" button> Click on the "Edit Parent Topics" to add the parent topic to this group.
When the group is added under a parent topic, community members will see the name of this group when they are visiting the topic page. Below is an example: