Minecraft Realms is a server hosting community for Minecraft Pocket Edition which allows players to create small servers with up to ten people from anywhere in the world. So far it is in Beta so only selected people qualify for servers. That will be changed around the time PE leaves Alpha and goes into Beta.
When you purchase your Realm (or server) you will immediately have full control over it. You decide the name, seed, and gamemode of the server you will be creating. Upon creation you have your Server Dashboard. On the dashboard you can decide who you want to whitelist (add people), to turn the server off or on, and many more details that will be added soon. When creating the server you wil be able to invite you friends to join any time even if you are not on. Within the server you get a normal world. You will be able to chat with the other people on your server which will help you be able to build things easier or just chat a bit.
How Do I Join Realms?
Evven though Realms isn't fully done you can still join it for future usage. Just click "Options" in the menu then select "Invited to Minecraft Realms? Login". Then click sign up, fill out the info then you're done.