MUX74HC4067 is an Arduino library that offers an API for interfacing with the 74HC4067 multiplexers / demultiplexers.
Multiplexers are devices that choose among a number of different inputs and channel that input to a single output. This way, a handful of sensors are now able to connect in turn to a computing device through a common bus. Demultiplexers work in the opposite direction. That is, they take one input signal and convey it to one of several outputs. They allow a computing device to use a single port to control many modules.
MUX74HC4067 simplifies the use of the MUX module and offers safe operation. That is, it makes sure that no signals get mixed up by always setting/resetting the right channel before anything is read or written to the signal pin.