JSON (JavaScript Object Notation)

JSON, or JavaScript Object Notation, is a lightweight, text-based data exchange format that human beings can read and write, and that machines can parse and generate. JSON is often used when data is sent from a server to a web page. It's derived from JavaScript, but it's compatible with many other programming languages.

JSON is often used with ReactJs because it uses the JSON structure to maintain the application's state, send HTTP requests, manipulate DOM, etc.

Here’s an example of what JSON looks like:

```
{
"name": "John Doe",
"age": 30,
"city": "New York"
}
```

In this JSON object, there are three key-value pairs. "name", "age", and "city" are keys, and "John Doe", 30, and "New York" are their respective values.

JSON is especially beneficial in ReactJs as it is a quick and efficient format to transfer data from the server to the client or in memory data store as application's state, allowing a swift and smooth data flow while maintaining optimal application performance.