Multiplexer is an msi device,These are called as sata selectors in terms of electronics .The basic function of these multiplexers is to select any one of the inputlines as output.Multiplexer is usally called as mux in electronics for simplisity.
Select lines are the input lines that are used to choose anyone of the input as output .
If there are 2 input lines and if we have to select any one of them then we use 1 select line,That is since select line can have either 1 or 0,at those two different logic levels different input lines are selected as outputs.
if there are 2 power n input lines then n select lines can be made to select any of them as out put .
Truth table for multiplexer
When I1,I2 are the input lines and S is the select line then
###At S=0 ,I1 will be selected as input from the two available inputs ,That is what ever may be the value of I2 I1 will be selected,the value of input I1 will be the output when select line is zero and I1 is zero output will be zero ,when select line is zero and I1 is oneoutput line will be 1.
$$$ At S=1,I2=O/P that is what ever the value of input I2 is applied to input terminal same will appear at output terminal of the multiplexer.