When a new WordPress site is set up it has with one category called uncategorized. You are supposed to rename it to something meaningful within your niche.
We will change "uncategorised" to "Blog" in the first instance, as nearly every site has a blog. You can always add more categories later, or rename the Blog category to something else.
WordPress has two basic places where you can place article content. These are posts and pages. The key difference between them being that posts have to belong to at least one category, whereas pages are not placed in categories at all.
Pages are designed for static administrative content (about us, contact us, privacy, disclaimers ... that kind of thing), whereas posts are supposed to build into a dated, categorised and catalogued repository of information. This post content will form the most important content on your site. Categories are therefore supposed to relate to your site’s niche. So a wine site, for example might have the following categories :
Never leave "uncategorized" as a category. It looks bad and also wastes a valuable SEO opportunity.
Surprisingly many people never rename the uncategorized category and the consequence is that all their posts end up in it. There are so many instances of this that uncategorized is now a major keyword. For example, if you type uncategorized London into google you'll get hundreds of sites about London that rank for this pointless keyword phrase.