GraphQL is a query language (and runtime) for building and using APIs: developers use it to simplify making API requests.
Most apps and sites are built on a network of interconnected APIs: when you load your profile or feed, they provide the data. APIs are generally built with a purpose-per-endpoint, which means that getting all of the data you need involves many calls and server trips. GraphQL acts as a wrapper on top of your APIs so you can say exactly what you want and only need to talk to the server once. GraphQL also supports schemas and introspection to make your data easier to understand.