The subtitle files (.srt) have a certain format like,

start time --> end time 

The dialogue . . .


And that repeats. The mediaplayers usually display 'the dialogue' 
between the 'start time and 'end time'. This app has a widget running 
with a stop watch. So you can start it with playing the movie alongside. 
When you want to add subtitle to certain scene you just mark the 
start time and endtime. You do it for all the scenes you need to it., 
and finally the app will save it in a file. You can type in the 
dialogues under those time marks then :)