Crit'Air is an abbreviation for certificat qualité de l'air = Air Quality Certificate.

The certificate is a round sticker that must be displayed in a prominent position on your vehicle. It is a secure document.
There are 6 different types of certificates.
Each certificate corresponds to a vehicle class defined according to air pollutant emissions.
Older vehicles covered by less stringent emissions standards are not eligible for a certificate.
This website is the official site for issuing Air Quality Certificates.

You can apply for your certificate (€3.11 + franking) here.

For more detailed information go to the official French site.