
I am really cofused. Thank you for your answer by now.

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.
Pandiatonicism in “She’s leaving home” (Beatles)?
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!