Subtitles and closed captioning work by layering text over the video so you can read instead of listening. iOS even lets you create your own display styles so you can make them as easy to see and enjoyable to read as possible.
Below are the instructions on how to turn on subtitles and closed captioning on your iPhone and/or iPad:
STEP 1: From your device’s Home screen, go to Settings.
STEP 2: Tap on General.
STEP 3: Tap on Accessibility.
STEP 4: Tap Subtitles & Captioning under the Media section, which you can find at the bottom part of the screen.
STEP 5: Tap the switch to turn them on.
STEP 6: Tap on Style in order to customize how closed captions work if you’d like.
STEP 7: Tap on Create New Style, then choose your preferred font style and size.