Click the little 'CC' button in the toolbar. Video authors can upload closed captions to videos, or Youtube will autogenerate using voice-to-text which, while isn't perfect, does a decent enough job.
Worth noting that this was a presentation at Texas JavaScript Conference, and the presenter likely had nothing to do with the video being uploaded to Youtube.
They are bad, but it is better than nothing. I think if you are completely deaf they probably are not good enough. For myself, I have some hearing damage from working with fire alarms without hearing protection which makes it sometimes hard to understand speech with some background noise, it also doesn't help that English is not my native language. These subtitles make some videos that are hard for me to understand much better than without, because I am able to combine the hearing that I got with the subtitles to figure out what is being said. So yea, I hope they could improve them, but without them my experience is definitely worse.
I think they're okay, given what they are. I've used them a few things to watch foreign language things. They're not perfect, but at least enough to understand what's happening.
Worth noting that this was a presentation at Texas JavaScript Conference, and the presenter likely had nothing to do with the video being uploaded to Youtube.