A body kept on a table experiences a downward force due to its weight. But the table exerts an equal and opposite force on the object to keep it at rest.
Due to the weight of the object, a force is exerted on the table equal to the weight of the object. The table in return exerts a reaction force on the object equal to the magnitude of the weight but opposite in direction (Newton's third law). Thus, the net force on the object is zero as two equal forces in opposite directions are acting on it and hence, the object remains at rest.