On a digital piano with either two or three pedals, the leftmost pedal is a soft pedal. It is used to soften both the volume and the timbre of the instrument sound. On Yamaha digital pianos it is a subtle effect designed to be used only on the softest passages. To demonstrate, repeatedly play a C note 2 octaves below middle C and alternatley step on and release the soft pedal. The effect is much more noticeable when playing sofly. On some instruments there is a Soft Pedal Depth parameter that allows the user to adjust the effectiveness of this pedal.
Do you need an additional help?