Double sided have panel wiring on both sides.
However, to use the wire of the both sides, must have proper circuit connected between them.
This bridge between the circuit is called guide hole (via).
Guide hole is full of or coated metal on the PCB, it can be connected to the wire on both sides.
Because the double sided PCB's area is twice of the single sided, and wires can be interleaved with each other (connected around the other side), it is more suitable for complex circuit than single sided PCB.