An expanding creative is an ad with two sizes, an initial collapsed size 
The smaller collapsed size fits within a standard ad placement on a website, while the larger size appears to expand outside the collapsed size. The ad can expand automatically or when a user clicks or rolls over, and you can control that setting when you build the creative.
This guide walks you through the process of building an expanding creative using Studio’s HTML5 SDK. To start building an expanding creative, see Add the HTML5 Enabler and follow the workflow in the side panel.