An advertiser is charged for viewing a TrueView in-display ad when someone:
shares the ad
clicks and views the first frame of the ad
watches the entire ad
watches a TrueView in-stream ad after watching a TrueView in-display ad
Your ad consists of a thumbnail image from your video with some text. While the exact size and appearance of the ad may vary depending on where it appears, video discovery ads always invite people to click to watch the video. The video then plays on the YouTube Watch page or on your channel page.You’ll be charged only when viewers choose to watch your ad by clicking the thumbnail.
Read more here: https://support.google.com/adwords/answer/2375464?hl=en