Dumbbells are simple, handheld weights used for strength training and fitness exercises. They usually come in pairs and are designed to improve muscle strength, endurance, and overall body conditioning. Dumbbells can be fixed-weight or adjustable, allowing users to choose different resistance levels. They are commonly used to train the arms, shoulders, chest, back, and legs, and are suitable for beginners as well as advanced athletes. Dumbbells are popular because they are versatile, easy to use, and effective for both home and gym workouts.