#Topics help organize Posts so everyone stays notified and up to speed on the things they care about. If you Follow a Topic, you'll get notifications when someone creates a new Post with that Topic.

Anyone can create a new Topic just by typing a '#' followed by the desired Topic name. 

Clicking on any clickable Topic embedded in a post will take you to the Topic landing page in it's host Community. Here, you can Follow or Unfollow the Topic by simply selecting the "Subscribe" or "Unsubscribe" button at the top right.
Additionally, you can also see who is following a Topic in your Community.

Also, from the left navigation, underneath your Subscribed topics, you can choose "See All"

This will bring up a window that will allow you to browse all of the Topics in a given community, and Subscribe or Unsubscribe to Topics. 

Community Moderators can create and delete topics by visiting their Settings page, accessed on the web at the right side of the screen when you click "Settings."

Once in the Settings, you'll find a section called "Moderation"

Open this section, and you'll see a button called "Moderate Topics."

This will show a list of the current Topics in your community. You can click the "✎" to edit the Topic or Description, the "👁" to view the Topic in context, and the "🗑" to remove the topic.

Clicking the "Add Topic" button from this screen also allows you to create new topics.

Clicking the check-box next to a Topic makes it a "Default" Topic. This means that everyone who joins the community from that moment forward will automatically be Following that Topic, and it will be available in the topic selector drop-down menu during Post creation.