Prior to the advent of the Apple II in 1977, a computer was usually built in a case or mainframe with components connected by a backplane consisting of a set of slots themselves connected with wires. The CPU, memory and I/O peripherals were housed on individual PCBs or cards which plugged into the backplane.
With the arrival of the microprocessor, it became more cost-effective to place the backplane connectors, processor and glue logic onto a single “mother” board, with video, memory and I/O functions on “child” cards — hence the terms “motherboard” and daughterboard. The Apple II computer featured a motherboard with 8 expansion slots.