Disappearing messages are automatically deleted after the selected time.

Enable disappearing messages in a chat
  1. Open the chat.
  2. Tap the name of the contact or group.
  3. Tap on Settings.
  4. Select Disappearing Messages.
  5. Choose the time: 8 hours, 24 hours, 7 days, or 90 days.
  6. Confirm.

Messages sent after enabling this will automatically disappear after the set time.