GraphQL is a relatively new way to build user interfaces and APIs for consumers. It's essentially a querying language backed by a strongly typed schema, making it easy to write human-readable requests to get exactly the data you need. Compared to REST, whose responses are dictated by the server, GraphQL queries place the power squarely in the client's hands.
However, GraphQL is really just a spec that describes an API. Many features one might expect from an API, like authorization/authentication logic, bi-directional communication, or rate-limiting, still need to be implemented. Recently launched, Slash GraphQL is a fully managed GraphQL backend that takes care of all the hard work of setting up a GraphQL API, letting you concentrate on building your app.