CORS stands for Cross-Origin Resource Sharing, which is a mechanism that allows a front-end client to make requests for resources to an external back-end server. In other words, it is a browser security feature that restricts cross-origin HTTP requests with other servers and specifies which domains can access your resources.

In Node.js, CORS can be enabled using the cors middleware package available through the npm registry. This package provides a Connect/Express middleware that can be used to enable CORS with various options. The cors middleware can be used to allow certain or all origins to request a resource from APIs by sending back a property in the response called Access-Control-Allow-Origin.

To use the cors middleware in Node.js, you can install it using the following command:

npm install cors

Then, you can use it in your Node.js application as follows:

const express = require(express);
const cors = require(cors);
const app = express();

app.use(cors());

This will enable CORS for all routes in your application. You can also customize the cors middleware by passing an options object inside the cors() method. Inside that object, you can specify the origin property, which allows you to specify which origins can access your resources.

const express = require(express);
const cors = require(cors);
const app = express();

const corsOptions = {
  origin: http://example.com,
  optionsSuccessStatus: 200
}

app.use(cors(corsOptions));

This will enable CORS for only the http://example.com origin.

In summary, CORS in Node.js is a mechanism that allows resources to be requested from external servers. The cors middleware package can be used to enable CORS with various options in Node.js applications.