These kinds of ads are non-interruptive which means they do not appear in videos people watch. Most people do not even realize they are clicking on these types of ads, which is a good thing. For the best results, discovery ads should be used to target customers in the middle of your sales funnel.
TrueView Discovery Ads appear either in the search results on Youtube, or at the top of the related video section. You are only charged when someone clicks on these ads, and the user is much more likely to watch your entire ad and make a purchase.