useState() is a Hook that is used for state management in functional components .  The  useState hook takes the initial state as an argument and returns an array of two entries. The first element is initial state and second is a function that is used for updating the state.

const [state, setState] = useState(initialstate)

To use useState you need to import useState from react 

import { useState } from "react" 

I'm making a button to increase the number. I will initialize with zero and on button click will increment one.

import "./styles.css";

import { useState } from "react";

const App = () => {

  const [count, setCount] = useState(0);

  return (

    <div className="App">

      <h1>{count}</h1>

      <button onClick={() => setCount(count + 1)}>Click to Inc</button>

    </div>

  );

};

export default App;