The braking system in your vehicle is a hydraulic system. The master cylinder has a reservoir of brake fluid which is moved through the slave cylinders to actuate the wheel cylinders. A brake slave cylinder is the piston and bore in your brake calipers. They are called slave cylinders because they're forced to work by the pressure exerted from the master cylinder.
Brake slave cylinders are flexible parts of the braking system which can be shortened or lengthened. Inside the brake caliper are pistons which press on the brake pads. The pistons are generally bigger on the front of the car because during braking this is where most of a vehicle's weight transfers. Calipers are smaller because there is less braking done in the rear, with less braking force needed. When the brake pedal is pushed down, a valve separates fluid pressure between the front and rear
Different Brake Slave Cylinders - Find the Right One