That means you can add web images to your profile, embed MP3 files, CSS formatted text (with hyperlinks, forms, tables) and even external Flash videos from YouTube and other video sharing sites.
While FaceBook doesn't permit the <embed> or <script> tags directly, the HTML box app provides alternate tags to embed these SWF or FLV videos in your profile.
For any Flash video file (.flv) <fb:flv src="Flash_Video_URL" height="###" width="###"/>
For Audio MP3 files: <fb:mp3 src="URL HERE" height="###" width="###" artist="artist" title="title" album="picture"/>
Facebook recently added the fb:google-analytics for Facebook app developers to track usage via Google Analytics. If you add an external web image to HTML Box, it will be cached on Facebook servers before serving it on your profile page.
Update: Facebook is no longer support HTML apps. You can however create a standard HTML file and upload it to Google Sites and embed it into Facebook. See how to create custom Facebook Pages.