Whenever you pick music on YouTube, instead of a 30 second long ad, instead you'll get a personalized intro for that song as it plays over the ad that acts as a build-up towards that song. Like if you're listening to Firework, and it just plays the opening notes over and over again, although sometimes a song will have a unique build-up if the song literally starts in a unique way like Bohemian Rhapsody having a new intro before the singers actually start singing. If you skip the ad, the song will start the moment the build-up loops.