The musical notation that indicates a note should be played softly is called "piano." This term is derived from the Italian word for "soft" and is represented by the letter "p" in sheet music. When musicians see this marking, they understand that the passage should be performed at a lower volume, translating to a gentle, subtle sound. Along with "piano," there are other dynamics in music notation, such as "forte" (loud) and "mezzo" (moderately), which help convey the intended expression and emotion of the piece being performed.