The Scenario module allows for creations of scenarios, which are "static worlds" with logic attached to them.
With the Scenario module a developer(or multiple) is able to utilize the HubTool item to create and edit logic and regions in the world around them, both in singleplayer and multiplayer.
Logic is composed of a list of triggers that all include customizable events, conditionals, and actions. Regions are areas that can be tracked to monitor players entering/leaving regions and for protection of areas of the world to prevent any editing.
Essentially, the Scenario module is a Game Maker created within Terasology that allows for altering the world logic around the player, while playing in that world. This allows for designers to develop scenarios to be experienced by other players!
Video tutorial about creation of a barebones fetch quest using the Scenario module, Here!
Some text tutorials about developing new trigger elements for the Scenario module.