Tap the Hidden album under the Other Albums section at the bottom. This is mostly used just to tidy up your primary Photo Library, so use it with caution.
The Photos app allows you to 'hide' photos from your Photo Library and any 'For You' recommendations, but it just gets tucked away into an unsecured album called 'Hidden,' which you (and anyone else viewing your device) can access anytime.
How to hide photos on iPhone in the Photos app
