What is sound check

It makes the volume between songs consistent. So if you are listening to some songs and one has much louder content, it will adjust it so it has a similar volume as the rest.

See:

Use Sound Check in iTunes to make song volume consistent - Apple Support

“Turn Sound Check on or off. Go to Settings > Music > Sound Check. Sound Check normalizes the volume level of your audio.” Excerpt From: Apple Inc. “iPhone User Guide for iOS 10.3.” Apple Inc., 2016. iBooks.

This article explains the Sound Check option:

Use Sound Check in iTunes to make song volume consistent - Apple Support