VAST stands for Video Ad Serving Template and it is specification released by IAB.
To make it easier to understand you can consider it as a video advertisement played in the video player before real content starts playing. So there is no big magic around it. It is something you meet online everyday (e.g. ads displayed on YouTube and other video websites). VAST has versions 1.0, 2.0, 3.0 and 4.0 and LiteAdServer supports VAST ads in the versions 2.0, 3.0 and 4.0.
To be able to display VAST ad it must be provided to VAST enabled video player. No each video player is VAST enabled.
It is worth mentioning that VAST ad and Video Ad are not the same type of ads although they are very often mixed.
VAST ad is displayed in VAST enabled video player before or during original content is played.
Video ad is standard mp4 video (sometimes with overlay) which is displayed in classical Ad zone (not in the video player).