Cryptography
Scrambling a message on the sender's device so that only the receiver's device can read it, not even the service carrying it.
Formal
A design in which data is encrypted on the sending device with keys held only by the people talking, so every server in between, including the provider's own, passes along data it has no key to read.
In plain English
Like posting a locked box that only your friend has the key to - the postal service carries it all the way but can never open it.
In practice
A ministry lets staff use a chat app with end-to-end encryption on their work phones. IT can manage the phones, but a request to the app's provider for the chats would return nothing readable.
Why it matters
Without it, every message can be read at the provider, so one break-in or one curious member of staff exposes everyone; the trade-off is that lost keys mean lost messages.