Files found on third-party sites can be infected with malware designed to steal personal data, spy on user activity, or lock the device (ransomware).
A "secret photo" app is a prime target for spyware. It may transmit your "hidden" photos and contact list to a remote server. download-secret-photo-telefonbuch-ipa
The Apple App Store reviews apps for safety. Sideloaded apps bypass this check, meaning they can contain malicious code. Files found on third-party sites can be infected
This inquiry appears to be about a specifically named, likely malicious or unofficial iOS application package file ( spy on user activity