The Digital Video Ad Serving Template (VAST) is a universal script for serving ads to digital video players.
It has been developed by the Interactive Advertising Bureau (IAB) and describes expected video player behavior when executing VAST-formatted ad responses. It provides detailed information about which ad to play, how the ad should show up, how long it shoud last, and whether users are able to skip it.
Basically, VAST lets video players and ad servers speak the same language. Standards mean scale, whcih means more volume for publishers to sell and more revenue.
The idea behind this script is actually to avoid forcing a publisher to change its video stack just to use different ad servers.
The latest version is VAST 4.0, which has some new features added and also supports viewability standards.