This one is pretty easy to understand, so we'll spend more time understanding actionable things you can do to be compliant on this mandate. Many people experience auditory difficulties that necessitate captions. So we can simply add subtitles to video (e.g. captions). Easy enough, right?
There are many different different file formats that YouTube supports. SubRip, SubViewer, MPSub, and LRC. If you're just beginning, try either SubRip or SubViewer. These can be edited in any basic text editor (e.g. Notepad). The format is really straightforward.
00:00:00,000 --> 00:00:04,160
>> JONAH: My name is Jonah, and I love sloths!
00:00:04,160 --> 00:00:06,770
>> BRITNEY: Well, who doesn't love sloths?
00:00:06,770 --> 00:00:10,880
>> JONAH: Only people that don't have souls.
00:00:10,880 --> 00:00:16,700
00:00:16,700 --> 00:00:21,480
BRITNEY: I know, right?!
How convenient and wonderful is this? All you need to do is type out what's said in the video, and the smart little goblins inside YouTube will automatically sync your text to the video. This won't be as perfect as the above method but this is still an effective way to provide subtitles to video:
If you're not an awesome person (and we know you are!), then there's an option just for you! Instead of taking time for an accurate representation of captions/ subtitles, you can simply have YouTube create the subtitles automatically based upon their voice recognition software. However, think about how many times Siri/ Alexa doesn't know what the heck you're saying? I speak quickly, I mumble from time to time, and will have a stutter after a couple glasses of wine. Machine speech recognition is awesome, but it's not 'quite there' just yet. This option is better than nothing, but I suggest taking a few moments to use one of the above two methods.
In this world of on-demand services, you can cheaply outsource this task to willing and competent people. Services such as Rev.com will do it for $1/ minute with a 24 hour turnaround time. Check them out here (disclaimer: we refuse any kickbacks for any recommendations/ affiliate links, so this comes from my dear, dear heart).