The WCS is the coordinate system for world space or the model space as a whole.
A book in object space rests on a table in world space. The table uses the world coordinate system.
World space is the universal coordinate system for all objects. World space is constant and immovable.
In the world coordinate system seen from the front, the X axis runs in a positive direction to the right, the Z axis runs in a positive direction upward, and the Y axis runs in a positive direction away from you.
The world axis shows the current orientation of the viewport with respect to world coordinate system. You can find it in the lower-left corner of each viewport. The world axis colors are red for X, green for Y, and blue for Z. You can toggle the display of the world axis in all viewports by turning off Display World Axis on the Viewports panel of the Preference Settings dialog.