It usually refers to soaps that:
* Contain actual flower petals or extracts: This gives the soap a beautiful appearance and a subtle floral scent.
* Are scented with floral fragrances: The soap may not contain actual flowers, but it smells like a specific flower or bouquet.
So, "savon à la fleurs" means that the soap has a floral element, either visually, in terms of fragrance, or both.