Minecraft Realms is a server hosting service introduced in 2013 to allow players to run multiplayer games easily and safely without having to set up their own. Only those who are invited can join Realms servers since they are not intended to be public Minecraft servers.
Moreover, these servers don’t use IP addresses. Realms server owners can invite a maximum of 3000 people to play on their server, with up to ten players online at one time. They do not support user-made plugins, but they do allow players to indulge in custom Minecraft maps.
User-made add-ons, resource packs, behavior packs, and custom Minecraft maps are supported as well. If you're wondering whether you need an Xbox account to join Realms or Realms Plus, the company says a free Microsoft or Xbox Live account is required.