How can I enable product filtering?

Shopify now provides native product filtering on collection pages. You can filter on availability, price, product type, vendor and product options. For more details on setting up product filtering, you can view Shopify's documentation here.

You can. Because sections are now available on all pages, including collection pages, the product filter options can be enabled or disabled for each collection template you use. Select the Product Grid section on any collection template to view the settings and to enable or disable filtering.

You can add custom filters using metafields. Check out this quick video for more details.