Adding categories and tags to posts and pages is completely optional.
Categories allow the broad grouping of posts topics when you want to describe a post. But when you want to describe a post in more specific terms, you would have to use or add more categories, or you can start adding tags.
To add categories that pertain to your post, scroll down on the “Write Post” page and click the “Add New Category” link under the “Categories” section which will then introduce a type and drop down box, along with an “Add” button. From there, start introducing categories for your post. For example, if you were writing a post entitled “Our Thanksgiving Dinner,” you may file it under the “Dinner” category.
Tags are similar to categories, but instead of a list of words to choose from, you write them in a list (separated by commas) in a text box below your post. They are free form words and generally describe your post in more detail. For example, like stated earlier, if your post was title “Our Thanksgiving dinner,” and you put it under the “Dinner” category, you could have tags like, turkey, mashed potatoes, family, and stuffing.