How am I able to click on "messenger" in the upper left menu of the website (2nd option down under News Feed) and able to send/view messages on my mac? I also g

How am I able to click on "messenger" in the upper left menu of the website (2nd option down under News Feed) and able to send/view messages on my mac? I also get an instant email notification if I'm not logged in when someone messages me so I don't need notification alerts.