Everythingclose to the Earth experiences the force of gravity close to theEarth. This force is called the weight of the object.
If the object is free tomove it will accelerate down because of the force of the Earth'sgravity acting on it. If an object is not free to move however – iffor example, it is on a table – the force of gravity does notvanish. It is there, and has the same magnitude and direction as ifthe table were not there. The object will experience the force ofgravity but remain stationary relative to the Earth, so there must bea force equality and opposite to the force of gravity acting on theobject.
This equal and oppositeforce is called the reaction force, labelled R above. The origin ofthe force is a bit more complicated. It arises from repulsive forcesbetween the electrons in the atoms of each surface.
Of course, a table cannotexert a reaction force to balance ANY weight. If the weight is tooheavy the table will not be able to support it and will break, butfor any lesser weight the reaction force will be equal to the weight.