CTR stands for Click Through Rate and it shows how many viewers click to watch your video after it is been shown to them in either: search results, home page, suggested video or notifications. The higher the CTR the more YouTube will push your video, because it understands that people are interested in your video.
How important is your CTR?
The better question is how important is your Thumbnail and title of your YouTube video, because actually these two are the drivers of the CTR. And to be clear they are the most important par. Your number one priority is getting people to watch your video, and you do that by an interesting thumbnail and title.
Next step is making sure those people stay as long as possible to watch your video. So after the CTR the WTR (watch time retention period is important). These two metrics tell YouTube several things, but the three most important ones are:
how interesting is the subject of your video
how good is the quality of your video, and
does the title & thumbnail represent the content of your video or is it click bait