MongoDB Stitch is a platform that allows developers to easily access MongoDB databases and integrate with key services. It provides native SDKs, integrated rules and functions to build scalable backends. Requests made through Stitch are parsed, services are orchestrated, rules are applied, and results are returned to clients. Stitch handles authentication, authorization and access controls through user profiles and declarative rules. It is a unified solution for building complete applications that connect to MongoDB and external services securely.