Is this your next interview question? ;-)

The term "Bit Banging" is used to describe a purely software implementation of a communications receiver and/or transmitter - without the use of dedicated hardware such as a UART, I²C Controller or SPI Controller.

For a transmitter, you have to write code to set each pin on the interface to the required state, and maintain that state for the required time, and with the correct timing relationship(s) to other pins, and do any necessary serial/parallel conversion;

For a receiver, you have to write code to sample the pin(s) on the interface with the required timing, and do any necessary serial/parallel conversion.

Obviously, this is an awful lot of work for both the programmer to write and the processor to execute.
The processor overhead means that it is only suitable for relatively slow interfaces.