A PCB is defined by the designer with a number of copper layers in a define sequence that is known as Buildup or Board Buildup.
The pattern in each copper layer combined with the drill patterns and the copper plating of these drilled holes, form an electrical net that is responsible for connecting the pins of components to other pins of other components.
This makes it possible that currents can flow and the application can function.
The Buildup or Board BUILDUP of a PCB is the sequence in which these copper layers are defined.
The picture shows a 6-layer board presented in our buildup wizard which is a part of the Eurocircuits calculator. Read also.