When you are submitting your video to YouTube, you can tag the videos main keywords and phrases. A few years ago YouTube took tagging to be an important SEO strategy but not so much these days but it still allows you to say to YouTube these are the topics of the video.
If you use Tubebuddy, the tool will make recommendations on the tags to add, as in the image below:
When you are tagging you have up to 450 characters, for example, you can tag as many keywords or phrases up to 450 characters. When tagging try not to spam, try to be specific to what the videos are about.