The tutorial focuses on using Clerk for securing API requests in Express or Node.js applications by implementing authentication middleware, enhancing API integrity, security, and reliability. By leveraging Clerk's ClerkExpressWithAuth() and ClerkExpressRequireAuth() middleware, developers can ensure that API endpoints are protected and only accessible to authenticated users. The tutorial provides a step-by-step guide on setting up an Express server, installing necessary packages like express, cors, dotenv, and @clerk/clerk-sdk-node, and configuring environment variables for Clerk's API keys. It also demonstrates how to test secured routes using Postman and integrate authentication into a React frontend, complete with user sign-in and token-based API call mechanisms. By following these instructions, developers can build a secure backend for React applications, utilizing Clerk's middleware to streamline the authentication process without the need for developing custom solutions from scratch.