Instead of writing a flat sign before every Bb note, it is easier to write just one flat sign after the treble clef. This means that all B notes on the staff are played as Bb, even though there is no flat sign placed before the note. This is called a key signature. One flat indicates the key of F major.
Play all B notes as Bb as indicated by the key signature.
This traditional Christmas song is written here in the key of F major as indicated by the key signature.
This D note is written on the fourth line of the staff.
This new D note uses the same fingering as F and Bb, which means that once again the higher note is produced by using more pressure with the lips. Here is an example which makes use of all three notes with this fingering.