The first most important (of the most important factors, lol) is your custom thumbnail (sometimes you get lucky and one of YouTube's default snapshots of your content happens to be an attractive image).
The thumbnail image must be clear, understandable, eye-catching, and must correspond greatly with your title.
Second most important for getting YT traffic is your title, which has two tasks - to inform about your video's content, and to grab the audience attention. The viewer's eye will very quickly glance over your thumb, and in most cases the decision whether to click or not to click on your video would take less than half of a second. Therefore, a bad, or irrelevant thumbnail and a dull title, devoted of vital keyword combinations will kill your traffic, regardless of you video's quality.
Keywords in title and description (tags, category, channel keywords are important as well) will place your video amongst others gravitating in the same tag cloud, and based on how often someone clicks on your video, YouTube will determine how relevant and attractive your video is to the audience. If your CTR is poor, your video will soon be forgotten, and would probably get a couple of views here and there.
If you manage to make your video clickable, YouTube will offer it to more, and more viewers until enough data is gathered to determine the quality of your content - how long does your video keep the viewer watching, and whether your video keeps the viewer within the YouTube website, either sending them to another of your videos, or to a different channel.
So
1 - THUMBNAIL
2 - TITLE (here I include description, there is a ton of info on how to optimize it)
3 - CONTENT
Provided you have done a decent job on task 1 and 2, the quality and relevance of your video's content will seal its fate.
If you have tricked your audience with a very attractive thumb and an amazing title (an offer we can't refuse), but the video content is of very bad quality or worse - has nothing to do with neither the title or the thumb...you're done. You will get a ton of dislikes, and despite your sexy CTR, YT will soon pull your video out of the traffic highway. If you mess up so much that you get the viewer angry, your video will get reported, and subsequently deleted.
The ideal case is to have all factors well balanced, so that your video gets clicked on, retains the audience, and moreover engages the viewer in such a way that they hit the like button, leave a comment, and subscribe to your channel.
If your videos manage to do this on a regular basis, you will become a YT favorite and will get rewarded with traffic. There is no secret pill or a shortcut to this game. It's common sense.
What you can do to get the ball rolling faster is to get your videos embedded on as many relevant websites as possible, and shared by as many social media accounts on different social platforms as possible. The more popular those sharing accounts are, the more viewers your video will get, and YouTube will love you for the fact that you bring in fresh viewers from other places, they lose their minds and spend a couple of unplanned hours on YouTube.