The term Multiplexing means "sharing of a resource/medium" .Resource could be time, frequency, space or any physical medium .Multiplexer is a communication device that multiplexes (combines) several signals for transmission over a single medium.
It helps different sources of data to use a single medium for transmitting their data.Basically the aim is to share an expensive resource
In more simple words, Multiplexer also called as Mux is a device that selects one of several analog or digital input signals and forwards the selected input into a single line.A multiplexer of 2^n inputs has n select lines, which are used to select which input line to send to the output.
A device that performs the reverse process is called a Demultiplexer (DEMUX).
Follow this tutorial for more understanding of Multiplexer
The Operation of a Multiplexer/Demultiplexer Circuit