Within a DAW (digital audio workstation) program, the terms fade-in and fade-out refer to smooth sound inputs and outputs.
Fade-in is the increase in volume (or any other effect), and fade-out, respectively, is the release (decrease). The shelves themselves can represent several variations that directly affect the speed and intensity of the processed audio track. This effect is included in the basic functions of every music program and can be used on any type of signal (regardless of format and duration). Similar volume rendering can also be created using automation or side-chain.
In sampled sounds (samples, loops), fade in and fade out represent the fading and fading effects of noise, reverb, and space. Used to emphasize dynamic transitions and logical construction of the structure of the composition.