This is a sample page which you can edit, delete or otherwise modify when you build your site. Pages are different to posts although they all seem to be the same to the reader.
A post is part of a series, rather like a newspaper article where the reader sees the latest post first but tomorrow it may be different. You tell the computer where you want to display all your posts when you post and the new post goes to the head of the queue and that is all you need to do. You can put a post into one or more categories then only posts in a particular category may be selected when setting up the menu.
On the other hand a page is fixed so in order for the reader to see it, it needs to be added to the navigation menu.