Multilayer PCB is a circuit board that has more than two layers. Unlike a Double-Sided PCB which only has two conductive layers of material, all multilayer PCBs must have at least three layers of conductive material which are buried in the center of the material.
Multi-layer PCB fabrication consists of laminating more than two conductive layers together and there can be various amounts of layers, depending on the application requirement. Having more layers inside the circuit board allows more circuitry and wiring for more complex applications.
You can likely guess what the significant difference is between these two types of PCBs based on their names. Single-layer boards have just one layer of base material, also known as a substrate, while multi-layer PCBs have multiple layers.