The latest Mastodon update lets you follow hashtags. When you follow, posts with that tag appear in your timeline automatically.
For example, follow hashtag #Florespondence to make posts about flowers appear in your timeline.
This feature can be REALLY useful for discovering interesting accounts.
To follow a hashtag:
1. Log in through your server's website
2. Search for hashtag
3. Click it to see its results
4. Click on ?+ icon in top right (click again to unfollow)
It's not on the apps yet.