`.srt` is the file format generated by the opensource software __SubRip]]. It is commmonly used for subtitles. For the wc3 standard for captioning video see: - WebVTT
__SubRip__ is a free software program for Microsoft Windows which extracts subtitles and their timings from various video formats to a text file. It is released under the GNU GPL. Its subtitle format's file extension is `.srt` and is widely supported.
Each `.srt` file is a Human-readable format where the subtitles are stored sequentially along with the timing information. Most subtitles distributed on the Internet are in this format- wikipedia 