The Custom Code page allows you to insert inside your app a non dynamic object by simply inserting some programming code (like html, css, js).
To insert the custom code page into your app click on the Content tab on the top from your dashboard:
and select the Custom Code feed from the left hand sidebar (under the General Content category):
Click on the Add Content button on the right:
and insert your code:
What are the best uses for a Custom Code Page?
With the Custom code Page you can integrate your app with features that AppsBuilder doesn't provide you by default (such as calculators or hyper text for example).