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.
You answered to my message but i have nothing in my notifications ; i would expect to have a notification at least for each reply of one of my post
