WindowClickSelect is an option for Notebook which specifies whether the window for the notebook should become selected if you click on it.
With WindowClickSelect->True, clicking on the window corresponding to a notebook makes that notebook the currently selected one.
WindowClickSelect affects selection of a window as a whole; Selectable affects only selection of the contents of a window.
See The Mathematica Book: Section 2.10.13.
See also: WindowFloating, SetSelectedNotebook.