Archimedes' principle states that an object submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. This means that if you place an object in water, it will push aside a certain amount of water, and the force pushing it up will be equal to the weight of that displaced water. This principle explains why some objects float while others sink. If the buoyant force is greater than the object's weight, it will float; if it's less, the object will sink. This concept is essential in fields like hydrodynamics and engineering.