A category is a useful way to sort and group related posts together in WordPress. All posts are filed under one or more categories.
You can choose to add several categories within the settings of your WordPress website.
When adding categories, you can also add a category description and specify whether you want to nest the new category which you are adding under an existing Top level category.
How to Add Categories?
You can add categories by navigating to Posts > Categories from your WordPress dashboard.
How to Change Default Category?
WordPress assigns a default category called ‘Uncategorized’ if you don't select any category while publishing your post.
You can change the default category under the Settings > Writing screen.