What is the reaction force? (eg from the ground or table)

The reaction force is the force that the table exerts on an object which is pressing on it.
The reason there is a reaction force is because the table is made of atoms and you can imagine these atoms been connected by really stiff springs. When the object presses on the table it squishes the springs slightly, and so the springs push back to try and return to their original shape. Its the springs pressing back which is the origin of the reaction force.