If you "follow" a toot, this page displays
The Tree checkbox allows you to switch between a hierarchical and a (purely chronological) flat layout of the replies.
Each toot has a checkbox, which you can use to tell the app that you have seen the toot. By default the contents of seen toots will be hidden ("collapsed"). But you can change that behavior with the Collapse menu, selecting whether no toots/only seen toots/all toots should be collapsed.
The header contains the following lines (below a list of links to other pages and the page title):
Each toot is displayed as a header with some metadata and a body with the toot's content. The latter may be hidden ("collapsed") as described above.
The ☰ menu of a toot can also be opened by right-clicking anywhere on the toot with the Control key held down.
The toot header also provides counts of boosts (B), favorings (F), quotes (Q) and replies (R). Boosts and favorings can be clicked to get more information.
If a toot has been edited, the creation date wil be struck out and the date of the last edit will also be displayed.
Finally: This is (for now) a read-only application. You cannot manipulate any data on mastodon instances. That is, you cannot reply, quote, favor, bnookmark, boost or edit a toot. Similarly, you cannot follow, mute, ... an account. Use the links to other mastodon clients and manipulate toots from there. On the longer run, I would like to have the functionality of this application integrated in a full-fledged mastodon app.