how can i find out the name of a brake part on my renault scenic?

i have a renault megane scenic (r reg) underneath the car at the rear in between the back wheels there is a metal box with four brake lines coming out of it and a spring connected to a rod which is fixed to the bottom of the car the spring is not tight its just spinning around it looks like some kind of hydraulic contraption i am trying to find out the name of it and what its usage is please can you help ps the car is a 1600 petrol renault megane scenic