GraphQL is without a shadow of a doubt the latest cool thing in the fields of providing a API. It’s not a total novelty, given that it’s been around since 2015 (publicly) and in theory, it’s some how running on Facebook since 2012.
In summary, GraphQL gives clients the power to decide what they want, including to fetch related data at - virtually - any depth level. How they do that? The answer is a query language instead of meaningless JSON payloads and blind REST like endpoints.