Bouyancy or upthrust is the upward force exerted by any fluid against the weight of the object immersed in the water or any other fluid.
Like when ever you try to immerse a plastic ball in the water,The water supplies a force and the ball comes back at you.
Factors on which the bouyant force depends:
(i) Volume of the fluid displaced.
(ii) Density of the fluid in which the body is immersed.
(iii) Acceleration due to gravity at the place.