It's a kind of check-up of your car !!
When do I need it ?
If your car is more than 4 years (then you need a « contrôle technique » every two years), or if it's more than 4 years and you want to sell it.
If you need to do one, search on the phone book (pages jaunes) or on the web « centre de contrôle technique ». You'll find the addresses and phone numbers.
You need an appointment and it costs around 60 euros (at least).
If your car is not in good condition, you'll have to repair it and pass a « contre-visite » which means a second « contrôle technique » and you'll have to pay again of course.
That's why some garages offer a « pré-contrôle gratuit ». It means they check your car,
tell you what is wrong, you repair your car and then take it to the « contrôle technique ». It's still expensive, but you pay only one « contrôle » instead of two...
Be careful, if you don't do this « contrôle technique », you can get a heavy fine or even have your car « immobilisé », which means you leave your car where you've been arrested, walk back home and pay a fine...
If you need a "contre-visite", you can still drive during a few months. It's written on your "carte grise" and the green sticker on your car window. So that during these months you can fix what's wrong and take a second appointment.