What does lifting state up mean in React?

Direct answer

Lifting state up means moving shared state to the closest common parent so multiple child components can read and update one source of truth.

When sibling components must stay synchronized, storing separate copies of the same state creates duplication. Move the state to their common parent and pass values and handlers down as props.

Accordion.jsx

function Accordion() {
  const [activeId, setActiveId] = useState(null);

  return (
    <>
      <Panel id="a" activeId={activeId} onSelect={setActiveId} />
      <Panel id="b" activeId={activeId} onSelect={setActiveId} />
    </>
  );
}