Double sided layout
The more complex a project gets the more difficult is it to route all tracks on one single copper layer without overlapping. Now you will have to use the copper top side (populating side, part side) for routing tracks. For leading tracks from one side to another one uses vias (a via is a plated through hole). During the process of placing tracks please press the full stop key [.] upon your keyboard to change the copper layer. A via is created and the track continues on the other copper side. Now you have a design double sided plated through (DSPth).
Creating a double sided PCB:
[F3] for getting to the PCB view -> [Strg] + [l]-> PCB-Layers -> choose "Standard layer set" -> double sided -> Set the layerfunction of layer 02 to copper bottom, and the layer function of layer 16 to copper top. When using the autorouter dialog allow routing upon these layers and define a preferred direction of routing (each orthogonally).