A lookup is a placeholder for a value that is not known until the workflow runs. Lookups can be used in many widget properties instead of defining a specific value.
When a widget uses a lookup, it retrieves the value stored in that location when the workflow runs (more specifically, when that widget runs). This allows a workflow to use a current value for an item so it can behave dynamically based on real-time data.
For more information about using Lookups, see Using Lookups in Workflow Studio.