It takes all posts that were posted on a certain day (based on your local timezone) and shows these posts ordered by their score. To keep it clean only posts with a score of 50 or higher are shown. (I might change this number though, if people would like to see less popular posts)

E.g. the main page is always 'yesterday', so for me it shows the posts that were posted between 00:00 and 23:59 on Wednesday, but if its already Friday where you are it will show the top posts of Thursday.