Yes, you might have to pay out of pocket for some things (special prescription eyeglasses, certain kinds of dental treatment, etc.), but the general idea is that you won't be hit with a surprise bill of several thousands because something went wrong.

Last year, my son (30 months old) spent a week in the hospital, underwent surgery, and had to keep a cast on for six weeks. The remaining costs, after the sécurité sociale paid its share, were 17€. Covered by the mutuelle.