This means the source of the file was a high-definition digital stream (likely from a legal streaming platform) which was then "ripped" or recorded into a file.
Be cautious when visiting sites like the one mentioned in your query. Such "free download" blogs often contain intrusive ads, malware, or provide access to pirated content which may be illegal in your region. For the best viewing experience and to support the creators, it is recommended to watch the film through official streaming services. southfreak-com-antardhaan-2021-bengali-480p-hdrip-x264-mkv
This is the "container" format. MKV files are popular because they can hold multiple audio tracks and subtitle files within a single document. This means the source of the file was