There are lots of accessibility options available in iOS that makes using an iPhone or iPad easier for those with visual and hearing impairments. If you or someone you know suffers from auditory issues, one of those features is subtitles and closed captioning while watching videos. To use them, you've just got to enable them first.
How to turn on subtitles and closed captions in iOS
Launch the Settings app on your iPhone or iPad.
Tap on General.
Tap on Accessibility.
Under the Hearing section, tap on Subtitles & Captioning.
Turn On the option for Closed Captions + SDH.
Tap on Style in order to customize how closed captions work if you'd like.
There are a few default options built in for closed captions and subtitles but if you don't like any of them, you can style on Create New Style... and choose your own font type and size. Just keep in mind that closed caption and subtitles will work where available but third party apps may have their own individual settings you'll have to use. This should get you started with all the built-in apps that need them though.