A Mux / Demux (Multiplexer / Demultiplexer) is a sort of traffic control device for electronic signals. It is generally used to provide multiple output paths for a single output pin or multiple input lines to a single input pin. Unlike many of the other chips that provide similar functionality, multiplexers are able to handle both digital and analog signals.

In this case, there are two common lines that have 4 sets of selectable paths, and therefore it is ideal for extending a Serial connection to allow for multiple input / output streams or I2C where multiple devices with the same I2C address are required. Of course, they are not limited to these two functions, they can be used with conventional digital or analog signals as well. To control which of the 4 paths the two common pins are connected to, a series of control lines are set HIGH/LOW.

These are ideal for microcontrollers / single board computers where there are limited pins available, but a lot of different control options / read options are required. In the case of limited numbers of pins with special functionality (interrupts, analog input / output, etc.) this can be expanded using a multiplexer instead of moving to a more complex controller.