You've probably done this: fetch some objects from an endpoint; then map those into a collection of keys that gets sent to another endpoint for more data; then hook all the results together in your client. This is stitching. GraphQL stitching simply automates this process so that one large supergraph schema can transparently query from many subgraph locations.
Stitching encompasses the raw mechanics of combining GraphQL schemas with cross-referenced objects, and is the underpinning mechanics for major federation frameworks such as Apollo Federation and Wundergraph. Stitching is generic library behavior that plugs into your server, while federation ecosystems give you a server, a schema deployment pipeline, a control plane, and opinionated management workflows.
If you're scaling a large distributed architecture with heavy throughput demands, then you'll probably benefit from growing atop a major federation framework. However, if you just have an existing Ruby app and want to hook a few external schemas into its GraphQL API, then incorporating an entire federation framework is probably overkill. Stitching offers good middleground.
This library contains the component parts for assembling a stitched schema, and rolls the entire workflow up into a GraphQL::Stitching::Client class.
For the most part, this entire library can be driven using just a Client. First compose a supergraph with executables for each location, configure its merged types, and then serve the client in your app's GraphQL controller.