Minecraft Realms are personal multiplayer servers, run by Minecraft, just for you and your friends. They're subscription-based Minecraft servers that allow anyone to setup Minecraft server without any technical knowledge.
Are Minecraft Realms servers free?
That really depends on server, Realms server hosting is not free, but Realms Minecraft servers are usually set up in a free to play way, you just join and you're good to go.
