Each time you launch the app you'll have the option to go through a tutorial, or skip it. This is one of the screens from the tutorial explaining what you'll find inside the app.
One thing left out of this part of the tutorial is that the words "Timer" and "Sounds" are, in fact, buttons and not text labels for the graphics below.
The main screen of the app provides a list of available sounds. Tapping on one opens a screen where you can view the associated photo and listen to ambient sounds.
Tap on the Play button to begin listening, or hold your phone up and move it around to view the rest of the photo.
You don't need to leave the app open to continue listening; it will play in the background.
The timer is perhaps my favorite feature of the app. Say you want to work on something for 30 minutes while listening to the app. Set the timer, and when it stops you know it's time for a break or to move onto something else.