A printed circuit board is a stack. At its core is an insulating laminate (usually FR-4, a glass-reinforced epoxy) with copper bonded to one or both faces. The copper is etched away to leave the traces and pads that connect your components. Everything else on the board exists to protect, insulate, or label that copper.
The layers you will hear about
On a simple two-layer board, from the outside in: silkscreen (the printed text and outlines), soldermask (the colored coating that insulates copper and stops solder bridging), the copper layer itself, then the laminate core, then the same in reverse on the bottom. Four-layer boards add two internal copper layers, usually a ground plane and a power plane, sandwiched inside the stack.
Why the stack matters for design
Every design decision maps back to this physical object. Trace width is limited by how much copper you have and how much current it carries. Vias are plated holes that connect layers. The soldermask defines where you can and cannot solder. Understanding the board as a manufactured stack, rather than an abstract schematic, is what separates a layout that can be built from one that cannot.