Angular momentum is a physical quantity that measures the rotational motion of an object. It depends on the object's mass, shape, and how fast it is spinning. For example, a spinning top or a planet orbiting a star both have angular momentum, which helps describe their motion in space. The principle of conservation of angular momentum states that if no external forces act on a rotating system, its total angular momentum remains constant. This means that if a figure skater pulls in their arms while spinning, they spin faster because their angular momentum is conserved, even though their speed changes.