If you want to easily follow a topic, to add more comments or read additional ones, click on the words Watch at the top left corner next to the Author's name. Then when you want to check on these topics, click on Watched Topics at the top of the page.
If you want to just save a topic for future reference, click on the words Bookmark, and then choose to keep them private or not, and click on save. When you want to view these, you click on My Bookmarks at the top of the page.