What does "fade out" mean?

Gradually disappear or become inaudible; also, cause to disappear or become inaudible gradually. For example, He let the final chord fade out completely before he played the next movement. The antonym is fade in, “to appear gradually or become audible,” as in The images on the screen faded in until they could be seen clearly. These terms originated in the motion-picture and broadcasting industries, where they apply to images and sounds. [c. 1915]

Also, fade away. Quietly depart, as in “Florence Scape, Fanny Scape and their mother faded away to Boulogne” (William Makepeace Thackeray, Vanity Fair, 1848). [Mid-1800s]