Buoyancy is the upward force experienced by an object when it is immersed in a fluid.
When an object is placed on the surface of water, it experiences two forces:
(i) Its weight which acts vertically downwards, and
(ii) the buoyant force which acts vertically upwards.
When the weight is more than the buoyant force, the object sinks while it floats when the buoyant force is more than the weight of the object.