As we know that a liquid contained in a vessel exerts pressure at all points and in all the possible directions and the pressure at a point in the liquid is same in all directions. it increases with depth inside the liquid. when a body, say a block, is immersed in a liquid, the pressure exerted upwards on the lower surface of the block(which is at a greater depth) is more than the pressure exerted downwards on the upper face of the block(which is at a lesser depth). thus there is a pressure difference between the lower and upper face of the block. since force=pressure x area, the difference between the pressures due to liquid on the two faces of the block causes the net upward force(i.e., upthrust) on the body.