Any Joomla based website consists of two sections: frontend and backend. Frontend web pages are what your site visitors view.
You can think of the backend pages of your website as of your virtual office. Here you or your team will create your site content and administer your site.
For security, to access a Joomla based site backend a user needs special permissions. If you are the one who installed Joomla system on your site, then you already have permissions to the backend as the site Super Administrator.