Question from the Next.js test

You create a page in your Next.js app in Page Router to display the details of a product. Here is a code snippet:

// pages/products/[productId].js
export async function getServerSideProps(context){
  const{ productId }= context.params;
  const product= await fetchProductDetails(productId);

  context.res.setHeader('Cache-Control', 'public, s-maxage=900, stale-while-revalidate=3600');

  return{ props:{ product } };
}

export default function ProductDetails({ product }){
//...
}

What function does getServerSideProps perform in this case?