How do array equals useState declarations work in Javascript?

In functional components within ReactJS, the following line allows the creation of:

1. A variable to record the current count
2. A method to save the state when called

Example in JS:

let [count, setCount] = useState([]);

How does this work from a declaration point of view and what is this kind of declaration called?

I understand basic declarations like let array = []; etc but am not familiar with the other way around.