What are Server Actions in React 19?

// app/actions.js
'use server'

export async function createPost(formData) {
const title = formData.get('title');
const content = formData.get('content');

const post = await db.posts.create({
  title,
  content,
  userId: await getCurrentUser()
});

revalidatePath('/posts');
redirect(`/posts/${post.id}`);
}

// app/new-post.jsx
import { createPost } from './actions';

export default function NewPost() {
return (
  <form action={createPost}>
    <input name="title" required />
    <textarea name="content" required />
    <button type="submit">Create Post</button>
  </form>
);
}

'use client'
import { useFormState } from 'react-dom';
import { createPost } from './actions';

export default function NewPost() {
const [state, formAction] = useFormState(createPost, { message: '' });

return (
  <form action={formAction}>
    <input name="title" required />
    <textarea name="content" required />
    <button type="submit">Create Post</button>
    {state.message && <p>{state.message}</p>}
  </form>
);
}

// This form works without JavaScript
<form action={serverAction}>
<input name="email" type="email" />
<button>Subscribe</button>
</form>