useState is a React Hook that allows you to add state to your components by returning an array with two variables: state, setState. The current state and the function that becomes the setter function when it is called. It can be used to track data or properties that need to be tracked in an application, such as strings, numbers, booleans, arrays, or objects.
Example:

const [state, setState] = useState();

In simple terms, state will change at some any point and it needs to be updated, therefore 'setState' will update the state, triggering a re-render of your components over time.

In addition, useState can hold any kind of Javascript value, including objects. Something to ALWAYS keep in mind is that you should never change objects that you hold in React state directly. First, you need to create a new one or create a copy of an existing one and then setState to the new copy. For example:

// Objects
const [state, setState] = useState({name: 'Marlo', age: 56});

const updateName = () => {
  setState({...state, name: 'Karlo'});
};

const updateAge = () => {
  setState({...state, age: 96});
};
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// Arrays
const [array, setArray] = useState([1, 2, 3, 4, 5]);

const addItem = () => {
  setArray([...array, 6]);
};

const removeItem = () => {
  setArray(array.slice(0, array.length - 1));
};

To use useState in a React component, first you need to import it form React by writing the following code in the top of the component's page in two different ways, both work perfectly so you can choose your poison.

import React from 'react'; 
import {useState} from 'react';

or you can write in one line

import React, {useState} from 'react';

React Hook useState can be called at the top level of a component or within custom hooks but not inside loops or conditions.

const [initialState, setInitialState] = useState();

the initialState is only used during the initial render and will be disregard in subsequent renders.