The origins of GraphQL stem from the needs that Facebook's mobile applications had (and continue to have). They needed a data-fetching API that was flexible enough to describe all the different kinds of data that the social network had available.
GraphQL is an application layer query language that interprets a string by a server, which then returns the required data in a specified format. You may have heard that GraphQL was invented to assist or enable Relay. This is false. GraphQL was actually in play nearly three years before Relay.