There are many factors in SEO, there being: Audience Retention, engagement, click spread (what I call it), shares, embedding/linking, titles description tags, and smaller miscellaneous factors.
Audience Retention. This one is probably the most important one on the list. If a user clicks on a 10 minute video and leaves after 30 seconds, then more do that in the future and the majority does that, YouTube will recognize it as a bad video and rank other videos that are similar higher. Since that video drives people away, it is unsuccessful. The fact that a lot of the time when some people find a bad video, they stop watching YouTube. YouTube knows this, so they enforce the videos that people actually watch. If a video has a high Audience Retention rate, YouTube sees that people like watching. People who are enjoying YouTube averagely watch more videos in the same session. You need to construct your videos in a way that will keep people watching for a decent amount of duration. Leave no dead spots that people will be bored of to leave. Watch every video you make. While watching, do not imagine it to be your video. Instead, imagine yourself to be a random internet spectator. Would you keep watching this if it were not your own video? What parts got boring as a spectator? What parts were the most interesting? Find these.
Engagement. This one is important too, and quite apparent as well. If a YouTube video has almost no comments and no ratings on it, then YouTube will think this video wasn't good enough for people to interact with it. Or even worse, it may think the video was view botted. So now other videos are place higher priority over this one. In your videos, have something to get people to comment and like. Ask them questions, tell them something they can relate to, tell them to like if they want to, etc etc. Something to get them to engage.
Click Spread (non-official name). Quite important and apparent as well. This means your videos/channel caused users to click on other videos of yours. YouTube does not want someone to click a video and hop off from watching. If your video caused someone to click another video, more importantly, yours, then the site recognizes that your channel causes people to continue to watch on the site. That means more likelihood of ad revenue for YouTube and use of their site. People don't even need to click to another video, as long as it sees that people click subscribe from your video, then YouTube will see this as good because people want to see content from you in the future. Make your videos to initiate Click Spread. Tell them to go to your channel page, subscribe, and/or click other videos. Construct the videos to make them crave more and more and more and more.
Sharing. Somewhat important. YouTube sees that if people share the video, then it is important enough for someone to show to other people. Same reason, it causes people to continue watching YouTube.
Embedding/linking. Important, not sure how much. If YouTube sees that many website have embedded or people click a link to your video, it shows that sites think you are important enough for them to spread it. If multiple important sites embed a video, not only is it raising the authority of the video (significance), you also will get views from that website. There is nothing you can do to initiate backlinking and embedding, just make it quality and hopefully websites will embed it.
Titles description and tags. Keyword everything, but do it intelligently with the title and description mainly. Construct your title to be quite relevant but containing a few different search terms. Use your description to actually describe your video, with keywords in it as well. Descriptions are very important, because if a website that has embedded your video has also used the description for it, people might actually read it more often on there than YouTube. Don't put tags in the description, because YouTube doesn't like it, and it hardly works. Just put in words that are relative to your video, or slightly relevant to your video while hiding them with sentences that actually help describe the video. Which is the way descriptions are supposed to be, the way YouTube wants.
After you have so & so amount of videos with decent video significance in YouTube's Algorithm, it will start assigning you channel authority, which means videos shortly uploaded which doesn't have time to gain significance already is boosted up past all the nonsense that has no authority. YouTube sees your channel as important because a handful of videos are important. It means that a lot of people like to watch your videos so a new one might do the same.
If you want me to go more in depth (there is only so far I can go, by the way) or you want some examples of good SEO, let me know.
I was a bit confused as towards where to place this 
