Nature sounds relieve stress from modern life.
The human mind reacts positively when it hears the sounds of nature because they evoke emotions that are reminiscent of our primordial environment.
Hearing the sounds of nature takes us away from the noise and everyday stress to make us return to the serenity of our origins.

You can use this app for sleeping, meditation, concentration or just relaxing.

Contains high quality relaxing sounds (all free) for any mood, divided into the following categories:

Rain Sound
Ocean Sounds
River Sound
Countryside Sounds
Night Sounds
Wind Sound
Fire Sound
Relaxing music
Oriental Sounds
Zen Garden
City sounds
House sounds
White noise, pink sound, brown sound
Binaural Tones

For a better experience, I suggest you use headphones or earphones to listen to relaxing sounds. Or you have to use some good external speakers, unfortunately the device speakers don't allow you to appreciate the sound quality.
You can keep apps in the background or turn off the screen so you can use your device for other purposes.