Although you can hide products on your online shop, it does not always stop customers from accessing them. For instance, if a customer had the web address for a particular product they can still access it because it still exists. The Hidden Products settings let's you choose what happens in the unlikely event that a customer finds a hidden product.
No Action To Be Taken
This option will allow the hidden product to be displayed if the product is accessed directly.
Hide Add To Cart Button
This option will allow the hidden product to be displayed if the product is accessed directly, but will hide the Add To Cart button, therefore, the product won't be able to be purchased.
This option will display "Product Not Found - The product page you are looking for might have been removed, had its name changed, or is temporarily unavailable." Displaying the 404 page will basically mean the page will drop out of Google within a few weeks at the very latest.