A video codec defines how frames are compressed and decoded. VP9 reduces file size by representing spatial and temporal similarities rather than storing every frame as a complete independent image.
VP9 is commonly carried in a WebM container for browsers. Transparent playback requires an encoding path and browser that support alpha data, so the final file should be tested in the actual target environment rather than assumed compatible.
ELI5
VP9 is a method for making video files smaller while keeping useful visual quality. It is often used for video delivered through web browsers.
For example, a transparent animation can be encoded with VP9 inside a WebM file for a website. The file may be much smaller than a production master, but the browser and encoder must both support the transparency correctly.
