User blog comment:Timeoin/Elder Scrolls Online: Lore Revealed/@comment-23610268-20130925164240/@comment-7153552-20130925165528

That would be in your shard or instance of the megaserver. So let's say you're on the Megaserver with everyone else, but they can't allow everyone to be rendered at the same time. This is for various reasons; resources, fairness in questing, cleanliness of the world around you, etc. To avoid said issues they will have separate layers of the megaserver, sometimes called shards or instances, and they were originally saying only 100 people per shard. They are apparently increasing that number in order to allow more people to play together at a time. This will also have PvP implications in Cyrodiil, I imagine.