This controls the edit interface you use to write your posts and pages.
Here you can set the default category for new posts. This means that every time you write a new article it will be automatically added to the category you set here.
So for this example, if you set the category to “Affiliate Marketing Tips” every new article you are going to publish will be categorized as Affiliate Marketing Tips. You can override this default category during writing/editing.
You can also set the Default Post Format here but this setting may not be necessary if you are still new to WordPress. The standard format will be enough for most users.