Upthrust is the upward force exerted on an object in a fluid. It arises as a result of the difference in pressure exerted by the fluid on the surface of the object at different depth.

Up thrust is the force that pushes objects up and makes them seem to loose weight in a fluid (note - a fluid means a liquid or gas)
When you are standing on the ground gravity keeps you down but upthrust stops you from falling into the earths core. In a way upthrust makes things hard!! Because if it didn't exist solids wouldn't be solid.An objects density effects how it floats.
Density = Mass
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Volume

Upthrust is also called buoyancy. It is the upward force produced on an object in a fluid (such as air or water). The force is equal to the weight of the fluid displaced by the object.
If that weight is greater than the weight of the object, the object will float in the fluid. If it is equal, it will sit suspended. If it is less, it will sink, but the object will weigh less than it would in a vacuum. Hence, it is easier to pick up a heavy object in water than in air - the amount of displaced water weighs more than the equivalent volume of air, therefore water produces a greater upthrust.