What is a resolver?

The FUNCTION that fetches the data for ONE field. The schema says WHAT exists. Resolvers say HOW to get it.

const resolvers = {
  Query: {
    user: (_, { id }) => db.user.findById(id),
  },
  User: {
    // runs for EACH user when the client asks for "orders"
    orders: (user) => db.orders.findByUserId(user.id),
  },
};

Execution resolves the selected fields, including repeated instances of an object in a list. A default resolver may read a property without a custom function. Directives, merged selections, and parent errors affect which fields run. In graphql-js-style servers the resolver receives (parent, args, context, info). Context can carry the current user and request-scoped loaders.