Each model has an internal, fixed coordinate system - the WCS, or World Coordinate System. The WCS cannot be changed, but you can define your own coordinate system - the UCS, or User Coordinate System. When you start a new drawing, the UCS by default coincides with WCS. All objects are defined and manipulated with respect to the current UCS.
Selecting an object using 2D Selector mode (see 2D - 3D Selector) changes the UCS (and therefore the workplane) to the one attached to the selected object.
When you move an object in 3D Selector mode, or an object that is part of a group or block, the UCS and workplane do not change. The Entity Coordinate System and its workplane travel with the object.
The UCS indicator is helpful if you want to see how the UCS moves. You can display markers for the UCS and WCS via the Local menu toolbars menu, and customize the markers' appearance via the Preference page of the Program Setup (Options / Preference). The WCS indicator is always shown at the lower left corner of the screen; the UCS indicator is shown at the UCS origin.