A baker's cyst is a fluid-filled sac that forms behind the knee, often due to swelling from conditions like arthritis or a knee injury. It occurs when excess synovial fluid, which lubricates the joint, accumulates and creates a bulge. While a baker's cyst can cause discomfort or tightness, many people experience no symptoms. It is usually diagnosed through a physical examination and imaging tests like ultrasound or MRI. Treatment may involve addressing the underlying cause, draining the cyst, or using anti-inflammatory medications to relieve pain.