A language server feature.
The tree view will display diagnostic errors, warnings, hint and information found in any of the open documents. Double click on an item to move the text cursor to the listed item.
Diagnostics are also displayed in the code explorer, but only for the currently active document. The diagnostics panel display information from all open documents.
Diagnostic errors, warnings, information and hints are displayed in the editor by a wavy underline and a colored maker in the left margin. Each diagnostic type has its own color.
If you hover the mouse over .isVerticals in the above code - you'll get a readable message in the hover hint.