A multiplexer is a combinational circuit that is given a certain number (usually a power of two) data with p inputs. Let us say 2^n and n address inputs used as a binary number to select one of the data inputs.
The multiplexer has a single output which has the same value as the selected data input.
In other words, the multiplexer works like the input selector of a home music system. Only one input is selected at a time and the selected input is transmitted to the single output. While on the music system, the selection of the input is made manually, the multiplexer chooses its input based on a binary number, the address input.