The news feed defaults to showing “top” items. You can toggle this to show the most recent items first by selecting “New” from the Sort By box underneath the left menu on any news feed page.
The Top setting sorts items according to a score, with the highest scoring item showing first. Scores are calculated by a formula based on the number of comments and votes a submission has earned, and the date it was posted. Preference is given to submissions with more comments and votes and those posted more recently. To enable new submissions to make their way to the top, the date of posting is given a relatively higher weight. As well, knowing some submissions earn significantly more comments and votes than others, there is an upper limit on the impact comments and votes can have on the score. So, a submission from several months ago with thousands of votes and comments won’t necessarily be displayed first.