"Other" is your stuff, and stuff for the apps you are using, and stuff that is not directly accessible and not otherwise categorized.
Per Apple: Other: Non-removable mobile assets, like Siri voices, fonts, dictionaries, non-removable logs and caches, Spotlight index, and system data, such as Keychain and CloudKit Database. Cached files can't be deleted by the system.
Your iPhone 6s is out of storage for what you're doing with it.
Since you likely won't want that to be the case, you can perform a backup and a factory reset and restore your backup, but—if this is the typical case—your Other usage won't have been appreciably reduced or will be rebuilt, and the storage dedicated to Other will be ~unchanged.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Restore your iPhone, iPad, or iPod to factory settings - Apple Support