Literal translation
to walk on sea urchins
Meaning
to be in a situation calling for great caution
Explanation
The local equivalent of "walking on eggshells". The sea urchin looms large in the Provençal imagination, both for its gastronomic value and for the painful memory shared by everyone who's stepped on one while swimming. It's the perfect image for something requiring great care, taken to the exaggerated extreme of actually walking on it. It's also sometimes used literally, for someone walking with difficulty over sharp rocks or scorching ground.