What is a data Source page?

The data source page is the screen you land on after connecting, where the data is prepared before any charting happens.

It has four areas: the connections pane on the left listing your sources; the canvas at the top where you drag tables and define joins, unions or relationships; the metadata grid listing every field with its type; and the data grid below showing a preview of the rows.

The work done here applies everywhere the data source is used — renaming fields, changing data types, assigning geographic roles, creating calculated fields, hiding columns you will not need, and setting extract filters.

Hiding unused fields before extracting is the cheapest performance improvement available in Tableau, and this is where you do it.