I just remarked that some of the questions on the main page are formatted differently than others. All questions which have been tagged with
biblatex have a yellow background. Is this a new feature or just a browser issue?
It may be that you favorited biblatex, as Martin explained, but it also can be that you just happened to visit a lot of questions with the biblatex tag: you can have implicit "frequented" tags. See this post on meta.SO for more details: Why are “etymology” questions on german.sx highlighted for me?
Questions tagged with your favorite tags are highlighted. It look to me that your favored biblatex.
Further to @Martin's answer: Tags can be favourited by hovering over the tag and clicking the star (the number of users who have favourited a specific tag is given in terms of the follower count):
Clicking the star again ignores posts with this tag (displaying the posts as almost being transparent/faded). Clicking a third time sets the tag back to normal in terms of its appearance.
Another way of accessing your tag favourites is under your user profile. Click on the prefs "tab" to view your "Favorite Tags" and "Ignored Tags". The usage and manipulation is very similar to adding tags to regular posts, and therefore should be straight forward.
To add to the other answers. Favoring tags is one way to watch and highlight certain preferred content on Stack Exchange. The two other ways to watch content that I can think of is favoring questions (i.e. your favorite questions) and subscribing to tags by either e-mail (the subscribe link in the hover of tags) or rss (the rss link in the hover of tags).