Invisible Ink is a feature in the Messages app on iPhones that allows users to send messages that remain blurred until the recipient swipes to reveal them. This feature works with both text and picture messages. To use Invisible Ink, users can follow these steps:

1. Open the Messages app on their iPhone or iPad and go inside a conversation.
2. Type a message they want to hide and send.
3. Touch and hold the upward arrow button that appears once they start typing in the chat box.
4. Tap the gray dots to preview different bubble effects.
5. Select the "Invisible Ink" effect.
6. Send the message.

The recipient will have to "wipe away" the blurry pixels to reveal the text. It is worth noting that the Invisible Ink feature works only with iMessage, and the recipient needs to have an Apple device to receive the message.