Polyfill

Polyfill in ReactJS represents a block of code that is used to provide modern functionality on older browsers that do not natively support it. ReactJS uses Polyfills to ensure that its newer features can still run on older browsers without causing the application to fail.

In other words, Polyfills enable developers to build their applications using the latest JavaScript features, while still providing backwards compatibility for older browsers.

A classic example of a Polyfill is the `fetch` API, a modern replacement for `XMLHttpRequest`. Not all browsers support the fetch API, so to ensure that it works everywhere, developers can use a Polyfill.

Here's a code snippet of how you can add a fetch polyfill:

```JavaScript
// First, you install the fetch polyfill
// Using npm
npm install whatwg-fetch --save
// Or, using Yarn
yarn add whatwg-fetch

// Then, in the file where you want to use fetch, you import the polyfill:
import 'whatwg-fetch'

// Now you can use fetch in this file:
fetch('/users.json')
.then(response => response.json())
.then(users => console.log(users))
```

This fetch polyfill (provided by `whatwg-fetch`) ensures that fetch works in all browsers. In browsers where fetch is natively supported, the polyfill does nothing. But, in those that don't support fetch, it provides an equivalent functionality.