The ABS hydraulic unit modulates the brake fluid pressure sent to each wheel. It sits between the master cylinder and the brake calipers. When the ABS computer detects a skid, it commands the unit's internal valves to release, hold, or build pressure. This stops the wheel from locking without driver input.
Internal valve wear or moisture in the brake fluid can ruin the unit. Moisture causes corrosion inside the small passages, leading to stuck valves. Bleeding your brakes every two years keeps the unit working.