Is there an account you already follow and you want to follow it more closely? Do you want to make sure you don't miss their posts?

You can do this on Mastodon's web interface by going to the account's profile page and clicking on the notification bell icon 🔔. When you have notifications switched on, you'll be notified every time the account posts.

You can also do this on Mastodon apps, but they may have different interfaces, such as a "Notify" button.

More info at https://fedi.tips/how-to-be-notified-when-someone-posts-on-mastodon