In a typical data selector the logic level on an output is the same as the logic level on one of the corresponding input pins. The selection pin(s) determine which one of the inputs is used.
For example, a select pin 'S' may be used to set the output 'QA'. If S is low then the output has the same logic level as input 'A0', if S is high it has the same logic level as 'A1'.