The access control list in the Drupal's admin panel is located under the People menu > Permissions tab. On this page you will see a list of check-boxes with the different types of users you have and the different actions they are allowed to take.
For example, you can add permissions for the different user types (anonymous, authenticated and administrator) to modify your site menus, or administer your content. All available actions are listed here, allowing you to setup your users permissions exactly the way you want.
A typical use of the access control is when you want to make some parts of your site available for registered users only. The default access control rules are quite safe, but you might want to change them for additional security measures.