Topic tracking is a forum feature that allows the forums to show a visually distinct topic title on forum topics with posts that you haven't read since your last visit.
To enable it, start by going to the forum page and clicking the "My profile" button near the top right of the forums. Next, click the "Board preferences" button near the bottom left. Lastly, next to the text that reads "Enable topics read" select "Yes" from the drop-down menu.