What does this product attribute base mean in WordPress?

This product attribute base refers to the URL structure that is used for products in WordPress. This URL structure can be customised to suit your needs, but the default structure is as follows:

/%product_cat%/%product_slug%/

So, for example, if you have a product category called “Shirts” and a product called “Red Shirt” within that category, the URL for the product would be:

/shirts/red-shirt/

You can change the product attribute base by going to Settings -> Permalinks in the WordPress admin area.