Search Engine Optimization is our way of trying to be more searchable. Basically we try to provide the information that the search engine looks for to know when our page, video, etc is a good search match.
You Tube is a giant search engine. All the search engines (including YT) will change their parameters to try and keep people from gaming the system. Basic SEO stays fairly constant, but they will trick it up now and again. Like YouTube tags. Tags used to be a major thing, now YouTube says they do very little. Keywords, description, title... are important... for now ha ha. It is an ever evolving battle to be searchable.
Back in my old HTML days we added tons of metatags to get searched (mid to late '90s). Them days are gone...