A backplane is a construct used to connect various components - usually each a single PCB - of a computer. The backplane contains a number of connectors, mechanically joined to each other, and then wired together; the components are plugged into the connectors. In early backplanes, the electrical connections beween the connectors were hand- or machine-wired (often using wire-wrap): later, PCBs were often used there, too.
The connectors often utilize contacts ('lands') on the edge of a PCB; when the edge is plugged into the connector, contacts in the connector touch the lands and complete the circuit.
In early computers, the components were small, and were only small sub-units (i.e. not a complete CPU or main memory unit); later, such entities were built on a single PCB.