Topics organize Quick Posts, Articles, and other activity like Polls, Questions, and Events by categories your Host defines. Right now, each post or activity can belongs to one Topic at a time.
By default, you're following the Highlights in every Topic. This is the default setting so that you can see the most popular activity across all Topics.
If you want to opt into seeing everything in a Topic, click the Follow button you see on the Topics page. Clicking Follow a second time will bring you back to the Highlights view of Topic. To completely Hide a Topic, read on.
Hide All Activity From a Topic
If you don't want to see any activity in a particular Topic, you want to choose to Hide Topic from the 3 dot menu you reach from the Topics page: