Progressive Download
A technique for downloading Internet video and/or audio clips so that they can be viewed at the same time that they are being transferred to your computer in other words process of digital media files that allows a user to access the contents of a file before the download is complete.
This provides some of the benefits of streaming media without requiring a special streaming server. The end user's experience is very similar to that of streaming media, and it is often unnoticed that the full file has not been downloaded.

For slow connections, progressive download is often the preferred method as it allows playback directly from the user's media player, thus eliminating the dependency on bandwidth.
Progressive download transmits a media file with metadata in the downloaded file's header. A media player capable of progressive download reads this metadata and starts playing the media file after a certain minimum portion of data has been downloaded
Media players also require a buffer to hold data for later use. The amount of data buffered prior to playback is determined by both the content producer (in the encoder settings) and additional settings applied by the media player. Prior to the development of the incremental download, the metadata referenced was at the end of the file. Because of this, playback was not possible before the entire file was downloaded.
For more information about media files check out our blog;
https://cinema8.com/blog/how-to-export-presentation-to-video