Listener is a roleplayer's power tool. It can range from being a simple addon to merely check what people are saying on an individual level, to a fully fledged chatbox replacement with a lot of flexibility for assisting dungeon masters. As of 2.0.0, the interface should be a lot more intuitive, so you can jump right into using it. Here are some things you might want to know:
Listener windows are basically advanced chatboxes. By default you just have one primary one, and you can close it out by clicking the X if you don't want to use it. The key feature of Listener windows is the ability to add or remove certain players showing up, making it easier to manage messages in a crowded setting.
The Snooper is a special listener window that works with whoever you're mousing over or targeting. It shows their text only. Right click the "Snooper" text to access configuration. Configuration can also be accessed by right-clicking the minimap button. A lot of the times, especially if you're new to Listener, the Snooper is the only thing you'll find yourself using (and can't live without).