Topics help you organize posts and see what people are
Using Topics to Organize Records
Enable topics for objects to allow users to add topics to records. Adding topics helps organize records and makes them easier to retrieve using list views. If your organization uses Chatter, topics help you link records to related conversations, content, and knowledgeable people.
Number-Only Hashtag Topics
To support the use of the number sign (#) in mathematical and other non-topic contexts, Chatter no longer automatically creates a topic when you type a hashtag followed by a series of numbers in a post or comment.