A stethoscope is the instrument used by healthcare professionals to listen to heartbeats and other internal sounds of the body. It consists of a long flexible tube connected to a chest piece, which houses microphones to detect sounds. When placed on the patient's chest, the stethoscope amplifies heart sounds, allowing clinicians to assess the heart's rhythm, rate, and auscultate for any abnormal sounds like murmurs or gallops. It’s a crucial tool used not only for cardiac assessments but also for examining lung and abdominal sounds, making it integral to a doctor’s diagnostic process.