https://www.vice.com/en_us/article/epzejp/how-to-turn-on-apple-icloud-encryption-game-changer

The cloud has always been a convenient place to store your files, but a hostile place for security. With your files backed up on a company’s servers somewhere, they are at risk to demands from authorities to access them, or hackers that may break into the company’s infrastructure.

Apple’s recently launched “Advanced Data Protection” offers to upend that idea by using end-to-end encryption for its iCloud service. This means that device and messages backups, notes, photos, voice memos, and more should be much better protected from third-parties when stored or synced across iCloud. The promise is that not even Apple will be able to access your data stored in the cloud, and by extension, third parties. This is because the keys for decrypting the content are stored on the user’s devices. This is a notable and welcome change; for years, Apple has encrypted data stored on individual devices, but often backups synced to the cloud did not have a similar level of protection.