Category is one of the two pre-defined taxonomies in WordPress that allows you to organize many posts of the same topic.
For example, when three posts are included in a category, they will be listed on the archive page for that specific category.
A post must have a category selected. However, it may belong to multiple categories as well. If no category is selected for a post, it will automatically be added to the default category.
If your website is new then ‘Uncategorized’ is the default category for all of your posts.
Also read: complete WordPress Glossary