SubRip caption file.

More on SRT file

SubRip caption files are named with the extension .SRT. This format is supported by most software video players and subtitle creation programs. The time format used is hours:minutes:seconds,milliseconds, with the milliseconds field precise to three decimal places.

SRT file does not contain any video data, it is simple text file you can edit by any text editor (translate subtitles into another language). The purpose of SRT subtitle file is to use it with desired movie file together to display captions or subtitles when you play video file by any player supporting SRT subtitles.