Ground substitution:

Here the act of performing an instantiation is a function like there is only one possible outcome means we can write it as a function. And the notation Subst({X/george, Y/tony}, as (X,Y)) = likes(george, tony)  it indicates that we have made a ground substitution. 

So now we also have to recognise that we are working with sentences that form part of a knowledge base of many such sentences. Just to move on the point we see there will be constants that appear throughout the knowledge base so some that are local to a particular sentence.