End-to-end encryption ensures that only the sender and recipient of a message can read its contents, with even the company operating the service unable to access it in transit.
iMessage and FaceTime use this model by default between Apple devices, though messages sent to non-Apple phones fall back to standard SMS, which lacks the same protection.
Encryption protects content in transit and often in storage, but it does not protect against a compromised device itself, which is why device-level security still matters just as much.