How do I make the "volume" of a sound get lower at the end?

Not sure if there is a name for this. I’ve got a sound with an abrupt ending to it. I want the sound to get quieter as it reaches the end, until its completely silenced. How can I accomplish this?

That is called a “fade out”.

First you need to “Select” the audio that you want to “fade”.
There are several ways to do that. The easiest way if you use a mouse, is to click and drag along the waveform. See here for other ways to select audio:
Then select “Fade Out” from the Effect menu.

Ensure that playback is stopped (not just paused) before you do this, Most commands are greyed out and unavailable during play, record and pause.