A multiplexer, or MUX, is a circuit that selects a single output from multiple inputs. It has multiple uses. The main use of a MUX is to select between input values, such as input values to the ALU in a CPU. But it can also be used to implement logic where the select lines are the inputs to a function, and the outputs to the function are hardwired to input values for the MUX.
A MUX is an interesting circuit as it actually contains a decoder circuit as part of its implementation. This allows the MUX to be more easily built using a decoder, and shows a valid use for a decoder.