In your own words what is the application (purpose) of macconkey agar

The Macconkey agar is both a selective and differential culture medium for bacteria. It selects only those that are gram-negative because the crystal violet and bile salts contained in it inhibit the growth of gram-positive bacteria. And it differentiates the gram-negative enteric bacteria based on lactose fermentation because these bacteria can ferment lactose and therefore be detected by a pH indicator that turns the medium into red.