A bleed nipple/screw is a part which is used to create a temporary opening in an otherwise closed hydraulic system, which facilitates the removal of air or another substance from the system through pressure and density differences.
Hydraulic braking systems
On a hydraulic vehicle braking systems, the bleed screws (sometimes known as bleed nipples or bleeder valves) are found at the top of each brake caliper to allow bleeding of the braking system. Whenever service work has been performed on the braking system which may have introduced air into the system (i.e. the hydraulic system has been opened up), the air needs to be bled out. This is crucial for correct operation of the system, because air compresses (as opposed to brake fluid which is not compressible). Air in the system decreases the maximum pressure applied to the brake pistons and can lead to loss of braking ability.
Hydraulic operated clutches
Cars having manual gearshift use a clutch. Clutches are controlled through pedal by bowden cable or hydraulic systems, some use the fluid in common with the brake system. As a dead end of the hydraulic system, the slave cylinder is has a bleed screw.