Its a text file containing time cues and the text to be displayed along with the associated video; here's the beginning of one:
1
00:00:40,880 --> 00:00:43,348
Help! Help!
2
00:00:43,560 --> 00:00:46,677
For God's sake!
Help!
3
00:00:55,640 --> 00:00:58,359
Get out!