Mewing is the technique of flattening out your tongue against the roof of the mouth. Over time, the movement is said to help realign your teeth and define your jawline.
Mewing involves keeping your tongue pressed to the roof of your mouth to change your jawline's shape. When someone tries talking to a person that is mewing, the person will put their index finger on their mouth (the 'shh' gesture) and then put their index finger on where their jawline starts and run it down their jawline.
ex. "Dude! Did you see John Doe's jawline in the grouppicture?"
"Yeah! I heard he's been mewing!"