Revenue is generated when a user clicks on an ad, when a non-skippable pre-roll ad is run prior to a video streaming, or when a skippable ad is played in its entirety.
If the advertisement is a non-skippable pre-roll, it will automatically generate revenue. If it is a skippable pre-roll ad, the user must watch the entire ad. If it is an overlay or display ad, then a user must click on one of the advertisements to generate revenue for you.
Types of YouTube Ads: