This is for advanced users, more for web developers that want to create spreadsheet-based mashups. A mashup is just a web page that takes data from existing sources and combines it into something new.
You create an Excel mashup by uploading a workbook to SkyDrive and then embedding it on a web page. Then, you use JavaScript to programmatically interact with that workbook.
I’m not going to explain here how to do this but just point you to the new site from the Excel team (http://www.excelmashup.com/) where you can learn all about it.