A metafield can hold a string and in this case the value is html code.
{{ product.metafields.spr.reviews }}
^ That code pulls the value of the metafield from the product and adds in to the page. The review app populates the metafield as needed so Liquid is able to grab it later.
The API docs for metafield will also be of use to you:
https://help.shopify.com/en/api/reference/metafield