How to add content to a main page
The default Drupal setup is to display a list of all nodes that have their "promoted to front page" box checked. 

If you want to display a single node rather than that list, you can create that page and then change the default front page in admin/configuration/site-information by pasting in that page's path to the front page field. 

If you would like to display a list that is different from the default list, (like, you want to add filters or display fields or something) then you can install and use Views to override the default or to create a new page and set it as the main page in site-information settings as described.