Core concepts

What is it

You can essentially do two things with GraphQL:

create a GraphQL server
consume a GraphQL server, either through a HTTP call or using a client like Apollo or Relay or any of the either multitude of clients available out there

There are some concepts being thrown around with GraphQL but really there isn't much to learning to use it:

Schema, this is the schema definition and pretty much describes the data structure the client can query, think of it as a database schema
Query, this is a question that returns a response
mutation, this looks like a question but CREATES, DELETES or UPDATES usually
Mutation, this function that will change something with your backend, compare it to an INSERT, UPDATE or DELETE
Resolver, this is a function that responds to something you are trying to do, it is part of your schema definition. Regardless of wether it's a query or mutation, it needs to have a resolver function that says "this is what's gonna happen if you invoke me"