You'll have probably noticed that at the end of a lot of pop songs, the music will get quieter and quieter until it's silent rather than suddenly stopping. This is called "fading-out", and the opposite of this, starting silent and getting louder and louder, is unsurprisingly called "fading-in".
Fading in and out isn't just used in music. It's a good way of making your audio sound a bit smoother. Listeners don't like sudden noises, and can cause your otherwise great audio sound quite harsh. An easy way to solve this is to quickly fade the audio in or out. To do this:
Select the section of audio you wish to fade in or out by clicking on where you want the fade to start and drag until the end.
Go to the Effects menu and select Fade-in or Fade-out.
That's it. Now enjoy your smoooooth sounds.