app.use(bodyParser.urlencoded({ extended: false })); is a middleware function in a Node.js application that uses the body-parser package to parse incoming HTTP request bodies.

The body-parser package allows you to extract data from the request body, which is useful when you're building web applications that need to handle form data or other types of user input.

The urlencoded method of body-parser is used to parse URL-encoded form data. When you submit a form on a website, the data is usually sent in the form of key-value pairs in the URL-encoded format, which looks something like this:

name=John+Doe&email=john%40example.com

The bodyParser.urlencoded() middleware function will extract this data from the request body and make it available as a JavaScript object in req.body.

The { extended: false } option in bodyParser.urlencoded() is used to configure the middleware to use the classic encoding algorithm for URL-encoded data. When set to false, the URL-encoded data will be parsed using the querystring library included with Node.js. When set to true, the URL-encoded data will be parsed using the qs library, which allows for more complex data structures to be encoded in the URL-encoded format.

In summary, app.use(bodyParser.urlencoded({ extended: false })); sets up the middleware for parsing URL-encoded form data in a Node.js application using the body-parser package.