Click-through rate (CTR) is the share of impressions that turn into views.
Impressions click-through rate in Studio = views from impressions ÷ impressions. If your thumbnail was shown 10,000 times and 450 people clicked, CTR is 4.5%.
CTR depends heavily on where impressions come from: search impressions usually convert better than browse impressions. Typical channels see somewhere around 2–10%. A low CTR is usually a title or thumbnail problem; test a new thumbnail before changing the video.