This extension flags unread respectively unvisited tabs by prefixing the title with one or more characters/symbols of your choice.
So you can see at a glance which web pages have not yet been read and find the corresponding tabs quickly.
In the options you can easily select a symbol for the marking.
To find other suitable Unicode symbols, you can visit e.g. the following pages:
Liste der Unicodeblöcke
Unicode 11.0 Character Code Charts