The GraphQL Schema Definition Language (SDL) is a core piece of building GraphQL APIs, allowing backends to abstract the makeup of backend services and frontends to validate and pre-ordain queries using introspection results. The SDL is a typed language for specifying various objects that can be accessed through a GraphQL API, with examples like `Post` and `Blog` types defined in a schema file. Prisma is an example of an SDL first tool that turns a database into a GraphQL API by letting users design their schema using the SDL, generating APIs for both the database and the GraphQL server. This approach allows for parallelizing product development, as seen in the deployment of a local Docker cluster to deploy a new GraphQL API from a Post SDL, enabling features like subscriptions and schema stitching, and can be used with existing backends by placing GraphQL over REST with tools like graphql-tools.