Instances are dungeons and special areas in World of Warcraft which many copies can exist at once. An instance allows many groups of players to traverse the same geography and battle the same enemies at the same time, without interfering with each other. Instances are accessible by a swirling portal. They may be blocked by a locked door which requires a key.
Instances were made as a way so solve a set of related problems which had become obvious. Everyone wants to slay the hardest monsters and get the best loot
, but having the whole server at the very same spot at the same time would've crashed the server.
In World of Warcraft you can find instances such as Naxx