Sometimes when you see a toot on mastodon, 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 a "toot tree" containing the replies given up to now. 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 quickly becomes tedious.
This application makes it more comfortable to "follow" a toot, or actually the replies to it.
See the "about" page for more information or go directly to the start page.