Each topic in a community can be a part of a more general topic. The more general one is "a parent topic", and the narrower one is "a child topic". If you follow a parent topic, all the child topics will be followed automatically. Also, if you go to a parent topic page, you will see all the questions related to the child topics as well.
To add a child topic to a parent topic, go to the topic page and choose the parent topic.
Once the parent topic page is up, click on the (...) button on the top right, then click on the "Details", and add the child topics and alias topics:
Subtopics or child topics will be added to the right side menu of the topic page.
Topic Aliases means similar topic. For example, "Tribe Referral" and "Tribe Affiliate Program" are the same topics. So all the questions related to this topic will be on one page. This is very helpful because when users search for alias topics, tribe searching systems connect them all to one topic page. This is how it works: