Opengl programming guide 8th edition says "OpenGL doesn’t include functions for performing windowing tasks". What are windowing tasks?

Creating the Window that GL runs in is operating system specific, the WIN32 API for Windows for example. GL is platform independent hence it doesn't implement the drawing of windows. Windows are are what you see in desktop apps, like dialog boxes or message boxes, they have a handle assigned to them and GL uses this to do the actuall rendering inside this windows handle.