Apps for Excel are programs that run inside (or along side) Microsoft Excel 2013 to extend its functionality. The following installation instructions are from Microsoft's site here:
Get an app for Excel
Apps for Office can help you personalize your documents or speed up the way you access information on the web. For example, with an app you could look up items on Wikipedia or add an online map to your document without having to leave Excel.
Click Insert > My Apps.
In the Apps for Office box, click Store.
Search for "Bank File Conversion" in the search box. When you find it click Buy.
Review the privacy information, and then click Trust it.
If you’re prompted to sign in, type the email address and password you use to sign in to Office programs. Review the privacy information, and then confirm billing information and complete your purchase
Start using your app
To start using your app, double-click it in the apps list.
Double-click any app to start using it.
Note After you get a new app, it should show up in your apps list. If it doesn’t, click Refresh to update the list.