What is React and its main features?

import { useState } from "react";

// You describe the UI for a given state; React updates the DOM.
export default function App() {
  const [on, setOn] = useState(false);
  return (
    <div style={{ padding: 24, fontFamily: "system-ui" }}>
      <button onClick={() => setOn((v) => !v)}>
        {on ? "🟢 On" : "⚪ Off"}
      </button>
      {on && <p>Declarative rendering in action.</p>}
    </div>
  );
}