When the jrEmbed module is active, this button is added to the tinyMCE editor.
Clicking on that button will popup a dialog in a new window of all the things from your jamroom site that can be added into the page being created.
This can be useful if your doing a blog about a song and want to include the song. Or for doing a sales page for a bundle of items and want to include the link to the bundle for sale.
Any module that allows its contents to be embedded will appear in the popup dialog, ready for the creator of the document to put it into the new page.