Bleeding means removing air from a hydraulic system.
When pressure is applied to liquid in a hydraulic system, the liquid does not compress into a smaller volume. Pressure is transmitted without loss. Gases however are compressible.
Pressure applied to air changes its volume, and some pressure is lost. That is why if air enters a hydraulic braking system, it can be dangerous.
Pressure on the brake pedal will not be transmitted in full through the system to apply the brakes. The brakes will be spongy.
Bleeding the brakes means removing this air, so that only liquid is left in the system.
