Perhaps you don't want subtitles to be on permanently. After all, if you're not using them, they can get annoying. But don't worry; it's equally as easy to manage subtitles on a video-by-video basis.
Once your video is playing, click on the Settings icon in the lower right-hand corner. In the pop-up menu, you'll see a list of all the subtitles files you have available for the video in question. Click on a language to activate the file.