SRT is the simplest subtitle format: a numbered list of cues, each with a `HH:MM:SS,mmm --> HH:MM:SS,mmm` timestamp and one or more lines of caption text. Originated in 2001 from the SubRip ripping tool and became the universal default — every video player (VLC, MPV, mpv, every web video framework), every subtitle editor, and every video platform reads SRT.
How to open
Any text editor (it's just text). VLC autoloads `<videoname>.srt` if it's next to the video file. Subtitle editors like Subtitle Edit, Aegisub, and EditSubs provide visual timing tools.