In MMOs all players are part of a persistent world, which is the same for each player and in which actions are to some extent permanent. When the player exits the game, the world still exists and continues without them, and can change while they are not playing. However, most MMOs also have areas which are separate from this persistent world, in which a new version of the area is created each time a player or group of players enters. This means that each player or group of players gets to tackle the area on their own without interference from others. This also ensures that each player or group gets a chance at the same loot without a rival player or team taking it first. These are known as instances. In some MMOs such as World of Warcraft, instances come in the form of ‘dungeons.’ These are areas that a group of players need to fight their way through, killing enemies that are harder than normal, and facing very difficult unique enemies known as ‘bosses.’ Other MMOs such as GuildWars separate all quests into instances, meaning that players can interact in cities and shops, but not when they are fighting enemies or completing tasks. This also seems to be the case with the Maze, as Rick states that all solos are instances, and earlier noted that his quest had an entrance. He also says that the Roots are not an instance. This means that while he is in them, he can kill or be killed by another player.
A video tutorial about dungeon instances in World of Warcraft: