The widget included in YITH WooCommerce Best Sellers displays best-seller products in the entire shop. With this widget, you can select two types of displays, one based on the number of sales, the other one based on publishing date.
Then there is the widget YITH WooCommerse Best Sellers Categories, the widget YITH WooCommerce Best Sellers Categories shows a complete list of all categories in the shop that contains at least one of the bestseller products.
In case you only wanted to show specific product categories (among those already existing in the shop) type them into the widget field Categories. Which you will be able to find under Appearance -) Widgets.