pcb layer stacking
To add to the explanation already provided -
PCB Editors have two ways of representing solid copper planes - positive and negative. A postive plane is one in which you place the copper you want with pads, fills, and poured polygons. A negative plane is one in which you place marks where you want to remove copper.
The advantage of the negative plane is much smaller Gerber files for board fabrication. For a positive plane, there has to be a Gerber entry for each region of copper, and Gerber photoplotters draw that area with tens of thousands of line segments. For a negative plane, the photoplotter only has to draw the smaller areas where copper is to be removed.
Every trace on a PCB is a transmission line. For maximum energy transfer and best signal integrity, the trace should have the same impedance as the source and the load. The impedance of a PCB trace depends on the width of the trace, the distance between the trace and the signal return path, and the relative permitivity of the dielectric between the trace and the return path. The signal travels down the trace you place on the PCB, but there always has to be a return path for the current. In a controlled impedance board, we carefully control the distance between the signal trace and the return path - usually by placing the trace on a layer of the board adjacent to a continuous solid copper plane.
Automatic component placement equipment for fabrication of circuit boards uses small cameras to align the equipment with the circuit board. A fiducial is a target spot that you place on your PCB so those cameras have a well defined object on which to focus. The global fiducials let you line up with the board, and the component fiducial lets the equipment more precisely line up with small component footprints.