GRJ's answer is absolutely correct. However, if keeping track of many different topics and the number of bookmarks should become unwieldy, another way to keep track of them is to click the Like (heart) button. Any topics or comments you've Liked can be easily found later by going to your own profile page. Then under your picture, click on the "Reactions" link. This will show you everything you've "Liked".
Also, if you want to be notified any time new comments are added to a topic, you can choose this too. To do so, go to the top of the topic's page and click on the blue bell icon to the right of the Topic Title. When it turns red, this indicates that you will receive an "Alert" whenever there are new posts in this topic.