World, or sometimes context, is an isolated scope for each scenario, exposed to the steps and most hooks as this. It allows you to set variables in one step and recall them in a later step. All variables set this way are discarded when the scenario concludes. It is managed by a world class, either the default one or one you create. Each scenario is given an new instance of the class when the test starts, even if it is a retry run.
For more information, see CucumberJS World documentation.