Access Control in Drupal CMS
The access control list in the Drupal's admin panel -> People -> Permissions is quite self-explanatory.
There you can modify the predefined permissions for the different user types (anonymous, authenticated and administrator).
A typical use of the access control is when you want to make some parts of your site available only for registered users. The default access control rules are quite safe, but you might want to change them to satisfy better your needs. Spend some time playing with the permissions options to better understand what you can do.