The short version: Each day you make a post. This will replace your existing post. If you don't write a new post within a day*, the current one will disappear. Every day you write without missing, you increase your “streak”, which is the number of days in a row you have written on the site. We also keep track of your best streak.
*OK, the one post a day isn't entirely accurate. There's a little bit of leeway because it would be a little unfair to demand you post exactly once every twenty-four hours. So, here are the rules:
The site genuinely does overwrite your last post each day, there is no archiving happening. If you write something profound and want to keep it, you should make your own copy.