What is a TrueView ad on YouTube?

TrueView is an ad format on YouTube available as either in-stream or video discovery ads.

With TrueView, your company can choose how you display your ads on YouTube. You also get to pick how — and when — you present your videos to your target audience. The targeting features of TrueView also eliminate the waste of irrelevant views.

Source: webfx.com, Google Help