What Express Actually Does
When a user visits your app URL, something needs to decide what to show them. When they submit a form, something needs to receive that data and do something with it. When your app needs to talk to a database or external service, something needs to handle that communication. Express is the something.
In plain language: Express is the traffic manager for your web app. Every request that comes in gets routed to the right place. Every response gets sent back correctly. Express handles all of this with simple, readable code.
Why you need it: without a server framework like Express, you can only build static websites -- pages that just display content. With Express, you can build dynamic apps that receive user input, store data, send emails, process payments, and call external services. Most real business applications need Express or something equivalent.
How Express Works in Your Replit App
Your Replit app typically has a file called server.js or index.js that is the entry point. Express runs in this file. It listens for incoming requests on a port, matches each request to a route you have defined, and executes the corresponding code. A route might be: "when someone visits /submit-form, take the form data, save it to the database, and redirect them to /thank-you."
Do You Need to Learn Express
No. Claude writes all the Express code. You describe the routes you need -- "when the form is submitted, save the data and redirect to /thank-you" -- and Claude writes the Express code that makes it happen. Understanding what Express does (traffic management) is more useful than understanding how to write it.