Think at topics as a way to (collectively) curate contents that are created daily over the fediverse.

A topic (eg. +transfeminism) can include any activity (of any kind and from any user) that fits its scope and may have sub-topics to fine grain the level of details of what it is about.

A user can follow a topic same as they would follow a user (in fact, at code-level, topic is an AP actor as well).

Anytime a new activity is added to it (by +tagging it), the topic will auto-boost it and that activity will appear on the timeline of any user that followed it (you will notice a little "published in topicname" above the activity).

When you create a topic, you can also define its boundary, and (soon) add roles - to specify which user has permissions to add / review / ... it.

Topics are a powerful way to collect activities that are trasversal to any specific user in ways that do not enforc the current social media trend of pushing users to become influencers.