It's the 'portal' through which your Extension gets access to its host's (ie. QlikView's) data.
The 'Data' array is created from the Dimension and Measurement tags you define in Definition.xml.
To control what each Dimension and Measurement hold, you can use the 'Initiate' tag in Definition.xml.
<Dimension Label="Dim1" Initial=""/>
<Dimension Label="Dim2" Initial=""/>
<Dimension Label="Dim3" Initial=""/>
<Measurement Label="Measure1" Initial=""/>
<Initiate Name="Chart.Dimension.0.Field" Value="City"/>
<Initiate Name="Chart.Dimension.1.Field" Value="Longitude"/>
<Initiate Name="Chart.Dimension.2.Field" Value="Latitude"/>
<Initiate Name="Chart.Expression.0.0.Definition" Value="Sum(Population)"/>
The above code comes from an Extension I created to use the LeafletJS library.
You can get the whole code from my GitHub repo in case you want to see how it works.