In SSRS, the window on the left is usually the Report Data pane, where you manage datasets, parameters, images, and built-in fields. It’s part of the report designer layout, and Microsoft documents that pane as something you can show, dock, or switch between tabbed and docked views.

What it shows

The left pane typically contains items you use to build the report:

  • Datasets.
  • Parameters.
  • Images.
  • Built-in fields.

That’s why it’s often called the place where you “see the report data” while designing a report.

If you meant the expression window

If you were asking about the expression editor used in SSRS, that’s different: it’s the dialog where you write expressions like =Left(Fields!Name.Value, 4). The Left function itself returns the leftmost characters from a text value.

In the designer

If the pane is hidden, you can usually bring it back from the View menu and select Report Data. Microsoft also notes you can dock it or dock it as a tabbed document depending on your layout preference.

TL;DR: the “window to the left” in SSRS is most often the Report Data pane, not the report itself.