Amazon has a variety of tiers for its Music services. If you're a Prime member, you automatically get Prime Music, which has millions of songs that you can listen to ad-free. If that's not enough, you can subscribe to the whole catalog with Music Unlimited. But there's another free, ad-supported plan that up until now, has been exclusive to those with an Amazon Echo.
Today, the company announced that the free tier is coming to iOS, Android, and Fire TV, for customers in the U.S., UK, and Germany. It doesn't give you access to everything though. If you don't have a paid plan, you'll be able to play "top playlists and thousands of stations".
Presumably, if you're using this free tier, you also have Amazon Prime, which means that you can access the songs that are available through Prime Music, and then you can also get access to the playlists and stations provided by the free tier. This can be helpful if you ask Alexa, which is included in the mobile apps and on Fire TV, to play something that's not on Prime.
Amazon also noted that if you want the full 50 million songs on Music Unlimited, you can get your first four months for $0.99.