Meaning:
The basic idea behind mewing is that you can change the shape of your jawline if you think about the placement of your tongue in your mouth. People also claim it can help with sleep apnea, sinusitis, and other conditions.
Source: Parents Magazine
https://www.parents.com/what-is-mewing-and-why-is-it-taking-over-classrooms-8598847
Further Reading:
https://www.webmd.com/oral-health/what-is-mewing
https://www.health.com/mewing-7098082