Your vehicles brake booster is designed to provide power assistance to the braking system. On most cars, the brake booster uses engine vacuum to do this. However, many modern cars use an auxiliary vacuum pump to provide vacuum to the brake booster.
The brake booster is in line between the brake pedal and brake master cylinder. It essentially amplified the force applied to the pedal by your foot and outputs more braking force in the hydraulic braking system. This makes braking your vehicle a much less physical task.
When a brake booster is failing you may notice that braking effort increases. the engine runs poorly or stalls with force on the brake pedal, or the relative resting position of the brake pedal has changed. In these cases it is recommended to inspect the hoses leading to the brake booster as well as the brake booster itself. If the brake booster is in fact faulty it is highly recommended to replace it as soon as possible.