It's the mutuelle. The way healthcare works here, the essential medical stuff is reimbursed by the universal healthcare system (sécurité sociale), and the rest is out of pocket. The mutuelle covers the part that's normally for you to pay, which can be quite significant for dentistry and glasses. My employer currently gives me a mutuelle that's decent to start with, and I chose to pay a 15€/month premium for increased coverage.

For example I just got a 900€ dentist bill recently. By default the sécurité sociale would just reimburse 300€ of it, for the essential care as well as a standard deductible on dental crowns. With my current mutuelle there's only 3€ left for me to pay.

That's a mandatory complementary insurance, the base costs are covered by the national insurance. It's a confusing situation that's somewhat similar to the US in that it's wasteful, pointless and mostly serves some private interests, but on a much smaller scale.