A flat sign lowers the note played by one semitone.
On the keyboard, play the key immediately to the left of the note to play a flat. When a flat sign is written on the staff it is always written before the note.
Two B flat notes (written as Bb) are shown in the following diagram.
These Bb notes are written on the third line of the treble staff and second line of the bass staff. The Bb note is the black key immediately to the left of the B note, as shown in the diagram.