Subtitles are glorified text files, each line has a timestamp, like

00:07:34,480 --> 00:07:38,080 Exterminate! Exterminate!

This tells the player to display the line from 7 minutes, 34 seconds, 480 milliseconds until 7 minutes, 38 seconds, 80 milliseconds.

If you want the subtitles to be earlier, you’re subtracting from those times so it’s minus, if you want them to be later, you’re adding so it’s plus.