Multiplexing is the process of transmitting multiple data streams, along with a of control segment and selecting one out many of those streams based on that control segment.
An example of a multiplexer may be implemented with this pseudo-code:
a = [
010010
110110
000100
110110
]
def mux(a, 0001), do: a[0] # 010010
def mux(a, 0010), do: a[1] # 110110
def mux(a, 0100), do: a[2] # 000100
def mux(a, 1000), do: a[3] # 110110