Display options to sort products in the current collection
When activated, a drop-down list of sort options are displayed in the options bar. Customers can choose to view products with various sort options including Featured, Best Selling, ascending and descending options for Alphabetically, by Date and Price.
The default sort order for each collection is set in your Shopify Admin under Collections.
Shopify Admin - Collection sort options
Customers can change the default sort order by using the Collection Sorting drop-down feature in the theme.