If you click the magnifying glass icon at the bottom of the official Mastodon app AND you leave the search box blank, you will be shown posts that are trending on your server.
Posts trend if they are newly published and lots of people are currently sharing them. The more boosts a post gets, the higher it appears on the list.
Everyone on your server sees the same trending list, it's not any kind of personalised algorithm. It's just a simple list of the most-shared recent posts.
Edited 8d ago