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.

How Does the Recommendation Algorithm Work?
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).
Want more control over your product recommendations?
Shopify’s Search & Discovery app will allow you to customise the products that are shown alongside your products. Any products added under the ‘Complementary products’ will be shown in the corresponding block on the product form. Any products added to ‘Related products’ will display in the ‘Recommended Products’ section
Learn more: To find out more about the Search & Discover app, click here to read Shopify's help pages.