Watchmen.2009.ultimate.cut.1080p.bluray.h264.aa... -

: The movie title and its original theatrical release year.

: The vertical resolution of the video (1920 x 1080 pixels), which is standard Full High Definition. Watchmen.2009.Ultimate.Cut.1080p.BluRay.H264.AA...

: The original source material used for the digital encode was a physical Blu-ray disc. : The movie title and its original theatrical release year

That string is a standard file naming convention used by digital media release groups to identify a specific version of a movie. That string is a standard file naming convention

: This is the most comprehensive version of the film, clocking in at about 3.5 hours. It integrates the animated Tales of the Black Freighter comic-within-a-comic directly into the Director's Cut of the live-action movie.

: The video compression codec used (also known as AVC). It is highly compatible with almost all modern media players, smart TVs, and computers.

: This is likely the beginning of the "release group" tag (e.g., AMIABLE, ARCHiViST), which identifies the community or individual responsible for creating and uploading that specific digital copy.