What is Realms? Mojang says:
Minecraft Realms is the easiest way to play Minecraft together with your friends and family. Realms worlds are always online and can only be accessed by an approved group of people. Though it's nice to have some alone time, there's something really special about creating and adventuring as a group. Realms makes that really easy to do.
When Realms goes live, it will support cross-platform play on the iOS, Android and Windows Phone Pocket version of Minecraft, along with the native Windows 10 version, via an Xbox Live account. Mojang added:
Yes - we have to keep the servers running 24/7, so Minecraft Realms is a paid service. We're not ready to share our pricing plan quite yet. You won't need an Xbox Live Gold subscription to use the service.
Mojang is currently allowing Android players of Minecraft Pocket Edition to download an alpha build of the game with Realms support to test it out. There's no word yet on when Realms will be officially added to the Minecraft mobile versions.