Multiplexing means transmitting a large number of information units over a smaller number of channels or lines. A digital multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line. The selection of a particular input line is controlled by a set of selection lines. Actually there are 2^n input lines and n selection lines whose bit combinations determine which input is selected.
As an example we will discuss about a 4-line to 1-line multiplexer. Each of the four input lines, I0 to I3 is applied to one input of an AND Gate. Selection lines S1 and S0 are decoded to select a particular AND Gate. The truth table (shown below) lists the input to output path for each possible bit combination of selection lines. To explain the circuit operation let us consider the condition when S1S0=10. The AND Gate associated with input I2 has two of its inputs equal to 1 and the third input connected to I2. The other three AND Gates have at least one input equal to 0, which makes their output equal to 0. The OR Gate output is now equal to the value to the value of I2. A multiplexer is also known as a data selector, as it selects one of many inputs and steers the binary information to the output line.