In Stream Advertising describes the ads that play at the beginning of a YouTube video, also known as pre-roll videos. They automatically play, although a user has the option to skip the ad after 5 seconds, so it’s important that it’s engaging enough that they want to watch to the end. Don’t fret if people don’t watch all the way through, as you’ll only be charged for users who watch to the end, making it less risky to test new ideas.