mv input.zip output.ipa

: Click on the resulting Payload.zip file and change the file extension from .zip to .ipa 0.5.2.

: AltStore is an alternative app store that allows sideloading on non-jailbroken iOS devices. When you sign in with your Apple ID, AltStore requests a temporary provisioning profile tied to your Apple ID, device UDID, and app bundle ID. The companion app AltServer runs on your PC or Mac, installing resigned apps via Wi-Fi or USB and refreshing them periodically to prevent expiration.

The conversion only works if the ZIP file contains a Payload/ directory at its root, and inside that directory, a .app bundle with an executable compiled for iOS.