The access control list in the Drupal’s admin panel -> People -> Permissions is quite self-explanatory.
To manage the access different users have to different areas of your Drupal site, go to People -> Permissions tab. In this page, you will see a list of checkboxes with the different types of users you have and the different actions they can make.
For example, you can add permissions for the different user types (anonymous, authenticated and administrator) to modify your site menus, or administer your content. Every possible action is listed here, allowing you to setup your site exactly the way you want.
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.