Stands for Blu-ray , meaning the source is a physical disc rip rather than a digital streaming (Web-DL) version. x264: The video codec used to compress the file.
This specific naming convention is a "scene" standard used to identify the quality and source of the file: The title and theatrical release year. 1080p: The resolution (High Definition, 1920x1080 pixels). subtitle Black.Widow.2021.1080p.BR.x264.AAC.6ch...
Since this is a Blu-ray rip, subtitles for other "BRRip" or "BluRay" releases of Black Widow will generally sync perfectly. You can find the specific .srt or .ass files on these major platforms: Stands for Blu-ray , meaning the source is