"Explore" is a really useful feature on Mastodon which lets you see trending posts and trending hashtags.

If you're using the official Mastodon app, click on the magnifying glass 🔍 at the bottom of the screen but do not enter anything in the search box. Tabs will appear that show trending posts ("Posts"), trending tags ("Hashtags"), trending links ("News") and the latest public posts on your server ("Community").

If you're using your server's website, click on # Explore or the # icon.