Product recommendations is a feature that you can enable on your product page going to Product page > Product recommendations > Show dynamic recommendations. At the bottom of your product page, a list of recommended products will be displayed to entice your customers to purchase more.
The algorithm that identifies recommended products is based on data about products that were purchased together and products with similar descriptions. It uses this data to provide a mix of those related products as recommendations for a given product.
In cases where purchase or product description data aren't available for a given product, products from a related collection are shown.
The algorithm uses the collection in the URL from which the request to the API endpoint originated, and pulls other products from that collection. If the product doesn't have a collection URL, then the recommendation algorithm finds collections that include the product and pulls other products from those instead (excluding collections with handles 'all' and 'frontpage').
To learn more about displaying product recommendations, click here to read Shopify's developer help pages.