In JavaScript, a closure is a function that has access to variables defined outside of its own scope, even after those variables have gone out of scope. This is possible because when a function is created, it forms a closure with the environment in which it was created, and it retains a reference to that environment.

Let's see an example:

```javascript
function createCounter() {
    let count = 0;
    return function () {
        count++;
        console.log(count);
    };
}

const counter1 = createCounter();
counter1(); // Output: 1
counter1(); // Output: 2
counter1(); // Output: 3
```

In the above example, we define a function called `createCounter` that returns a function. The function returned by `createCounter` is a closure. It has access to the variable `count` defined in the outer function, even after the outer function has returned.

Each time we call `counter1`, the value of `count` is incremented by 1, and the new value is printed to the console.

In other words, the closure remembers the environment in which it was created, and it retains a reference to that environment and the variables defined in that environment.

Closures are useful in a variety of situations that we will explore in the next few chapters.

## Code Challenge

Write a function called `fibonacciGenerator` that returns a function that each time it is called returns the next number in the Fibonacci sequence.

The Fibonacci sequence starts with 0 and 1, and each subsequent number is the sum of the previous two numbers: 1, 1, 2, 3, 5, 8, ...