bell icon lacks explanatory title text when hovered

When I enter a chat page - e.g. https://virtual-assembly.hubl.world/messages/@https~@~_~_api~!virtual-assembly~!hubl~!world~_users~_pierrebouviermuller~_ - I notice an icon looking like a bell at the top right. I don't know what it does, so I hover over it to find out, but no clue is given - not even in french ;-)

I would expect objects to all have a textual title, which would (on a conventional browser with a conventional pointing device) appear when hovering over it.