Have a look at the template for the product page:
wp-content/plugins/woocommerce/templates/content-single-product.php
‘woocommerce_after_single_product_summary’ is a point in the template code where extra code can be inserted.
The second parameter in the add_action() function is the name of the inserted new function containing the extra code.
The third parameter in the add_action() function is the priority. The priority becomes important if there is more than one bit of extra code to go in. The lower the priority number, the sooner the extra code is run.
The template shows us that the function ‘woocommerce_output_product_data_tabs’ is run at priority 10. You wanted your shortcode to run before that, so we use say a 5 which comes before the 10.