According to Google adwords team :
These ads pull assets from your YouTube channel, can use uploaded images, or your Google Maps listings to build full screen experiences within a standard Interactive Advertising Bureau (IAB) ad size. The ability to integrate content from other Google products – like Shopping listings, G+ pages – is coming soon. Because these ads are built in HTML5, using responsive design, the assets dynamically fill the publisher site, mobile app, screen, or device — desktop and mobile.
They’re billed on a cost-per-engagement (CPE) basis. The optimization engine behind Lightbox ads learns over time so your campaigns get better at reaching the audience most likely to engage with your message as your campaign runs. Like all of Google’s display ad offerings, Lightbox ads work seamlessly with our audience solutions and can be measured using Brand Lift reporting.