Pushletters are like newsletters which you wish to send your subscribers (on a specific time) all at once. Unlike AutoPushes, which sends emails based on subscription date, push letters are sent to all the existing subscribers at one at a specific date and time.
Here is a quick video tutorial that shows how to set-up push letters and send them to your push subscribers:
*Note* - A push letter once delivered cannot be edited. You can anytime re-use it though.