VAST, or Video Ad Serving Template, is an XML-based format that is used to serve video ads to digital video players. VAST is designed to enable the playback of video ads in a consistent and predictable way, regardless of the player or platform being used. VAST provides a standard set of tags and attributes that can be used to describe the ad, such as its size, duration, and other relevant details, as well as a set of APIs that can be used to interact with the player and track the ad's performance. VAST is supported by many popular video players and platforms, and is often used by ad networks, ad servers, and other ad technology companies to deliver video ads to digital publishers and advertisers.