What you should already know first...
Before graphql you had to create multiple endpoints and many connections to fetch all your data. Multiple trips to a library to read different pages from different books. Finally you need to glue them in the frontend.
With GraphQL you have one endpoint and you aggregate all the requests really deep.
How? You need to ask the whole thing that you need first, without going multiple times to the library. To do that you have to be a little bit more clever asking everything in the beginning.