Ads. Yes, if you own a Google Home, or any other smart speakers, you'll now be able to stream ad-supported YT Music without paying. From Youtube's official blog:
https://youtube.googleblog.com/2019/04/enjoy-youtube-music-f…
If you already have a Google Home, navigate to Account Settings in your Google Home app, tap Services and select Music. Then, select YouTube Music as the default music service. If you are setting up a new Google Home speaker, choose YouTube Music as the default music service during the setup process.
Music fans can now listen to free, ad-supported YouTube Music on smart speakers in the United States, Canada, Mexico, Australia, Great Britain, Ireland, Germany, France, Italy, Spain, Sweden, Norway, Denmark, Japan, Netherlands, and Austria. It’ll be available in more countries soon.
Like the free version of Spotify, you will not be able to request specific songs but rather only genres, styles or mood of music and Google will play a station based on that request. So no, you won't be able to play Despacito (so sad) on command until you subscribe to Youtube Music, which can be as low as a few dollars a month…
Not sure if this is any helpful since Spotify does practically the same thing but at least now you'll have other options, but personally I'll be sticking with Spotify as they have a better music recommendation engine.
Amazon have just launched similar service in us yesterday for Alexa speakers (for those who do NOT subscribe to prime)