Definition
Android APKs and AABs are signed with a keystore. iOS builds are signed with an Apple certificate and provisioning profile. Windows and macOS executables can be signed with a code-signing certificate. The signature covers the files in the package, so a modified file or a different signer no longer matches. Stores verify it before install and on update.
Relevance
A signature is how a store and a device know an update came from you and that the package was not changed after you signed it. An update signed with a different key is treated as a different app, and the store rejects it.
Example
You ship an Android update signed with your keystore. The store rejects a second upload signed with a new key, because the certificate no longer matches the app already published. A changed file inside the package fails the same check, because the signature covers the files.
Connected
Synonyms
Code signing, App signing