Here is the “long” description from the readme.txt file included with the plugin:
Have you ever wanted to add a personal comment into the body of a post or page and have it stand out from the rest of the content?
Have you really wanted to throw a rant in a review because the subject just really got under your skin but you don’t want to dramatically disrupt the content?
This plugin will allow you to style sections of the post, or page, content with a shortcode that can add more emphasis by leveraging a style element from the active theme.
These asides can be left open as part of the content flow; or these asides can be closed to leave your readers the option of opening them if they choose to.
… and you can find more details on the plugin on its page here.