A multiplexer is a circuit which selects between the input signals depending on select signal. In basic form of multipexer (2:1 mux) selects between two signals. Below fig represents the 2:1 multiplexer , depending upon the sel signal y will represent the input signal a or b

Fig: 2:1 mux

A multiplexer can be designed using logic gates. As the multiplexer is used more frequently in digital desgin, chisel provides the function called MUX

val results = Mux(sel , a, b)

where a is selected when sel is true, otherwise b is selected, type of sel is a chisel Bool. The inputs a and b can be any chisel base type or aggregate (bundlers or vectors) as long as they are same type

A Bundle to group signals of different types. A Vec to represents an indexable collection of signals of the same type