You can control when 1999 publishes your posts.
By default an item is published as soon as it's first posted.
When it's published it appears on your blog's home page and in your RSS feed.
You may want to change the default. To do so, choose Settings from the System menu at the right edge of the menubar, and click on the Misc panel. Uncheck the box next to Automatically publish new posts? and click OK.
Now, to publish you have to explicitly do it for each post. There's a command in the popup menu that publishes the item it applies to. Before publishing it confirms that you want to publish it.
The default makes the most sense for new users, so they don't have to find the Publish command to have an item appear in their feed. But if you're using 1999 regularly you probably should turn the auto-publish feature off, so you can finish editing your posts before they appear in the feed.