Yesterday, a story in The Atlantic alleged that the Trump Administration accidentally added their editor, Jeffrey Goldberg, to a Signal group chat discussing a military action in Yemen. If you were to take this abstract description of a chat app and add end-to-end encryption to it, what you end up doing is encrypting messages with the app such that only your friends can decrypt them, using the app on their own devices. This is in contrast with transport encryption (which protects messages between your app and the channel, and also from the channel and your friends’ apps, but not from the channel itself) and at-rest encryption (which protects messages on your device when you’re not using it).