In our Search results, you have various sort options above the list. One of them is 'Most Popular'. That sort option is based on a system maintained metric called Radioactivity.
Let's view a YouTube Video as an Introduction to Drupal 7 and the Radioactiivity Module
So, what we've learned is that the Radioactivity score of a page is increased by every page view. Over time the score will slowly decrease. If folks stop looking at a page, it will fall lower and lower in the list of popular pages.