An file is a "multimedia container" format. Unlike a codec that compresses video data, a container "wraps" various types of content into one package.

While natively supported by Windows 10 and later, it often requires third-party players like VLC Media Player or Elmedia Player on other systems. Critical Security Risks

While "C19" is not a widely recognized title for a single major film or television series, downloading any file with a extension from unknown sources carries specific technical characteristics and security risks you should be aware of. Understanding MKV Files

It can store an unlimited number of video, audio (including multichannel), and subtitle tracks, as well as metadata like chapter markers and menus.