Embers are posts you share which burn out and dissappear after a while. They can be made anonymously and once they burn out they are gone forever.
You can customizes how long an Ember survives, how many people they will be shared with, and how close others need to be to read them. You can also choose if you'd like people to be able to reply and if others as well can read those replies.
Aside from replying to Embers, you can start an anonymous, private one-on-one chat which remain open even after an Ember burns out.