Serverless functions in MongoDB Stitch are written in JavaScript ECMAScript and can be called from other scripts or external JavaScript leveraging the client SDK. They utilize a context object that provides access to pre-configured third-party services, values, users, functions, and utilities. The context object enables developers to easily access resources such as databases, collections, and authenticated users, making it possible to create rich, powerfully integrated applications without boilerplate code. MongoDB Stitch offers a powerful development paradigm for serverless functions, allowing developers to create better applications faster.