The Storefront Behavior section of the Product Editor includes an option to hide a product from display in the storefront. If this option is enabled, the result is a hidden product.
Hidden products are not displayed in search results, category listings, vendor listings or the list of "featured products" on the storefront's home page.
However, if a customer follows a link directly to a hidden product's storefront page, then they will be allowed to view and purchase it.
For example, if product SID 123 is hidden in my store, customers will never see this product when they browse my storefront.
But, if I provide a customer with a link directly to the product's details page, such as www.mystore.com/product-p123.php, then the customer will be able to follow that link, view the product information and purchase it from there.