State with useState

useState gives a component a piece of data that persists across re-renders, and a function to update it.

Example: a counter, and why it actually updates on screen

import { useState } from "react";

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

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Add</button>
    </div>
  );
}

useState(0) returns two things: the current value (count, starting at 0) and a function to change it (setCount). Calling setCount(count + 1) doesn't just change a variable quietly in the background, it tells React "something changed, please re-run this component and update the screen to match." That re-run-and-redraw step is the entire mechanism behind every interactive React UI, from a simple counter to a full form.

Why you can't just reassign a normal variable instead

let count = 0;
count = count + 1; // this changes the variable, but the screen never updates

A plain variable change is invisible to React, nothing tells it to re-run the component. useState's update function is what actually connects a data change to a visible screen update; that connection is the entire reason it exists instead of an ordinary variable.