Buoyant force is the upward force exerted by a fluid on an object submerged in it. This force occurs because the pressure in a fluid increases with depth, causing the bottom of the object to experience greater pressure than the top. As a result, the object is pushed upward, which is why objects like boats and balloons can float. The principle of buoyant force is described by Archimedes' Principle, which states that the buoyant force on an object is equal to the weight of the fluid it displaces. This principle helps explain why some objects sink while others float, depending on their density compared to the fluid.