Occasionally you see a toot on mastodon and you are interested in the (direct and indirect) replies to that toot. Somebody might have asked an interesting question and you want to see the answers given by other users. Or somebody makes a joke and you expect funny replies. Or ...
Mastodon clients display the current "toot tree". If you want to see future replies, you can open the toot in a separate browser tab and reload it occasionally. But then you have to search the tree manually for new toots, which soon becomes tedious.
This application makes it more comfortable to "follow" a toot, or actually the replies to it.
If you want to follow a toot, copy its URL to the field "URL of a toot to follow" in the overview page and add it to your list of followed toots. (This could be made more convenient if your favorite Mastodon client provided a button or link to automate this. But you can get similar convenience by using the bookmarklet provided on the configuration page.)
Select one of the list entries to see the corresponding tree.
Initially all the toots in a tree are in state "unseen" and will be displayed completely. You can mark toots as seen by checking the checkbox, which hides the toot body. Newly incoming toots will be "unseen" and will thus be displayed completely. This way you find new replies quickly without the need for searching through a tree again and again.
Actually the previous paragraph just describes the default behavior. It is possible to change what is being displayed by the menus in the header of the tree page.
Important parts of the application state are persisted by your browser:
This state data survives the closing and re-opening of pages and even a browser restart.