I am really cofused. Thank you for your answer by now.
The dot on the 3rd line is a note head: B♭ (no length defined) and the flat ♭ assigns that there some B will be flattened but we don't know where if we don't see the whole song.
Thanks to the posting of NewGuitarMan we know now the B will be flatted in the end of the song (2nd last bar) song will B♭.
(B♭ but not the entire song!- that's why the assignment is in brackets)
So this melody isn't in F and neither in Dm. Probably this is a good example of the new term I've learnt recently: Pandiatonicism.
There are several performances of this song but any of them is exactly identical with the one we can see the sheet music here.
The 'bemol' - flat in English - on that middle line denotes all the B notes in the piece will be B♭, (si bemol). Putting it possibly in key F or D minor. Or more likely, I guess, a mode of F.
Why it has brackets, don't know. Possibly as a courtesy reminder. And why there's a note head (dot) as well, still don't know!