React: What is React 19 useOptimistic?

1import { useOptimistic } from "react";
2
3function TodoList({ todos, addTodo }) {
4  const [optimisticTodos, addOptimistic] = useOptimistic(
5    todos,
6    (state, newText) => [
7      ...state,
8      { id: Date.now(), text: newText, done: false, pending: true },
9    ]
10  );
11
12  const handleSubmit = async (formData) => {
13    const text = formData.get("text");
14    addOptimistic(text);
15    await addTodo(text);
16  };
17
18  return (
19    <div>
20      {optimisticTodos.map(todo => (
21        <div key={todo.id} className={todo.pending ? "pending" : ""}>
22          {todo.text}
23        </div>
24      ))}
25      <form action={handleSubmit}>
26        <input name="text" />
27        <button type="submit">Add</button>
28      </form>
29    </div>
30  );
31}