What is the exact meaning of “data graph” and “data graph platform” in GraphQL and Apollo prespective?

I am learning about GraphQL and Apollo , i can say that i got most of the concepts easily, i am learning it from the docs, but i am still stuck with some terms that overwhelmed me a little

In Apollo docs, they are saying:

A data graph sits between application clients and back-end services, 
facilitating the flow of data between them:

An Apollo data graph uses GraphQL to define and enforce the structure of this data flow.

I can’t relate this to graph data structure 100%, so i am still confused:

1 - What is the exact meaning of a data graph sits between client and backend?

2- What is the exact meaning of data graph here?