What does it mean by the preservation of edge end point function in the definition of isomorphism of graphs?

Since you know that we are looking for two functions (Suppose one function is “f” and other function is “g”) which preserve the edge end point function and this preservation means that if we have vi as an end point of the edge ej then f(vi) must be an end point of the edge g(ej) and also the converse that is if f(vi) be an end point of the edge g(ej) then we must have vi as an end point of the edge ej. Note that vi and ej are the vertex and edge of one graph respectively where as f (vi) and g (ej) are the vertex and edge in the other graph respectively.