Chest physiotherapy refers to a group of therapies used to mobilize pulmonary secretions and includes techniques like postural drainage, percussion, vibration, and breathing exercises. It aims to drain secretions from the lungs using gravity by having patients assume different positions. Key techniques involve percussing and vibrating areas of the lungs in specific postural drainage positions to loosen mucus, followed by coughing to clear the airways. Chest physiotherapy requires proper training and is used to treat patients with excessive secretions or retained mucus in the lungs.