WooCommerce 3.3 added a new "uncategorized" category that is now required as a default category. This means any products without a category assigned to it will be automatically assigned to this default category.
You can rename the category, and I recommend doing so and using it. But if you would like to hide the default category from your main shop page, I wrote a small plugin to help with this. Just download, install, and activate. No configuration required. If you're more into writing code snippets yourself, here is the snippet the plugin is based off of.
Note that this category will always be a thing from this point forward, so it is best to make use of it rather than try to hide it. There are quite a few places it will be seen/used, aside from just the shop page. Here are a few of the places listed.
That said, you can hide it from the main shop page using the above plugin as a temporary fix whilst preparing to use the default category and/or sorting it out so all products have a category.