What is the significance of having two flats in a key signature and how does it affect the overall tonality of a musical piece?

Having two flats in a key signature indicates that the musical piece is in the key of Bb major or G minor. This affects the overall tonality by giving the piece a darker and more melancholic sound compared to keys with fewer flats. The presence of two flats also influences the harmonic and melodic structure of the piece, creating a distinct mood and character.