What is error.js in Next.js and how does error handling work in the App Router?

// app/dashboard/error.tsx
'use client';                          // error boundaries are Client Components
export default function Error({ error, reset }: { error: Error; reset: () => void }) {
  return (
    <div>
      <p>Something went wrong: {error.message}</p>
      <button onClick={reset}>Try again</button>
    </div>
  );
}